Both Captain America's shield and Thor's hammer Mjolnir are two of the most iconic weapons in the Marvel Universe, and very few superheroes get the chance to wield just one - but the list of characters who carry both into battle is shockingly low. Those who use both shield and hammer belong to Marvel's most exclusive club, and it is exceedingly rare to use both in combat. A list of every superhero (and villain) who has used both weapons is quite short, but all have proven themselves to be capable warriors in times of extraordinary crisis.
Note that alternate timelines, parallel universes and other media are mentioned below, since the list would be even shorter without their inclusion. Additionally, not every weapon is technically the same: Captain America's shield and Thor's hammer were both destroyed and rebuilt in the past (the broken shield often foreshadowing the arrival of a horrifying crisis). The list will also only include characters who've used the shield and hammer together; those who have used one and then the other years later are not listed below.
Captain America's shield may be made of vibranium, but any ordinary human is perfectly capable of lifting and using the weapon; the same cannot be said of Thor's hammer. Mjolnir is protected by an enchantment, and only those who are "worthy" may lift the weapon from the ground and use it to summon lightning or simply strike their foes. While the magical rules can be changed (by Odin, as seen in the recent Hulk vs Thor: Banner of War event), this list does not include those who overcome the enchantment.
Captain America
Captain America is perhaps the most obvious example of a shield-slinger, but the shield has changed drastically over time. Steve Rogers carried a simple metal shield into battle during the Golden Age of Comics beginning in 1941, but switched to his famous circular shield not long after. As for lifting Thor's hammer, Avengers: Endgame contains the most famous example of Steve Rogers wielding Mjolnir, but he actually held it far earlier in the comics.
In Thor #390, Thor fights alongside Captain America against a massive group of soldiers. In desperation, Captain America reaches for Thor's hammer, and to his astonishment, lifts it as easily as he would lift his shield. He quickly tosses the weapon back to Thor, but this marks the first time in the comics in which Captain America wielded the legendary weapon.
Doctor Doom
Doom is the only major villain on this list, but Marvel no longer believes he fits the villain mold; Doom has slowly turned from an out-and-out world conqueror into a reclusive, domineering dictator who prefers to stay in Latveria (when he isn't making a bid for more power). Even so, Mjolnir would hardly consider Doctor Doom a worthy wielder, considering all the murders he has committed in the past (to say nothing of his many human rights violations as a Latverian dictator). Fortunately for him, this Doom is only half of his former self.
In All-Out Avengers #2, the story begins as Doctor Doom leads his fellow Avengers while retreating from the Dark Tide. The story begins at the ending, and Doom is (slightly) more considerate of others, and even doubts his own abilities at one point. This is because Doom was split into two characters: the "Light Doom" who has heroic attributes and the "Dark Doom" who only seeks power. Stripped of his evil self, it appears Doctor Doom can hold both the shield and hammer, showing that a true hero is buried underneath his evil self.
Tigra
Tigra is a rather odd character to appear on this list, but her inclusion is a presumptive one based on a mysterious cover recently revealed by Marvel. Greer Nelson is not as well-known as Captain America or Doctor Doom, but she still has quite a bit of comic book history, dating back to her debut year of 1974 in Giant-Sized Creatures #1 (although she appeared two years earlier under a different name). The half-human, half-feline hero has enhanced speed, strength and agility, as well as unusual empathic powers.
In an upcoming issue of the Defenders Beyond series, Tigra is clearly seen holding aloft Captain America's shield with Mjolnir in her other hand. Given the nature of the Defenders' threats and adventures, it's entirely possible this Tigra lives in an alternate universe (the odd appearances of the other characters on the cover seem to confirm this theory). Even so, Tigra is far from the most obscure Marvel superhero to hold both the hammer and the shield at the same time.
Crusader
Multiple Marvel superheroes are parents to children (the Fantastic Four's Reed Richards and Sue Storm, the Incredible Hulk, and the Scarlet Witch and Vision to name a few), but Captain America has never had a child in the mainline 616 continuity. However, he's had children in alternate universes - and in What If? #114, readers were introduced to Crusader, the daughter of Steve Rogers and the X-Men member Rogue. Inheriting powers from her mother, Sarah Rogers has also inherited his ability to wield Mjolnir from her father.
In an alternate timeline in which the Battleworld from the Secret Wars crossover was never destroyed and the heroes remained on the surface, an uneasy truce formed between the Avengers, X-Men and villains remaining on the planet. When Victor Von Doom's son attempted to take over Battleworld, Sarah Rogers led her young Avengers in a successful attempt to stop the villain. Gaining Mjolnir in the process, Captain America's daughter used it to great effect - and just like her father, no one was surprised when she proved herself worthy.
Captain Marvel
Captain Marvel is already an obscenely powerful superhero, but holding Captain America's shield and Thor's hammer makes her practically unstoppable. But while Carol Danvers is a morally-pure hero (relatively speaking), she's also the only character on this list who isn't quite worthy of lifting the hammer. Instead of passing Mjolnir's test of worthiness, the hammer was tricked.
In Captain Marvel #16, Carol Danvers is captured by the villain Vox Supreme and hooked up to a machine that grants her the DNA of all the Avengers - including Thor's. Carol escapes and reaches out for the hammer, which flies into her hand. The hammer, believing Captain Marvel to be Thor, is used to make short work of Vox Supreme. This issue was rather controversial among Thor fans, who took issue with the hammer's inability to tell the difference between a DNA-doused Carol Danvers and the real Thor - but she was able to lift the hammer nonetheless, earning her a spot on the list.
Superman
If ever there was a DC Comics hero worthy of Mjolnir, it would be Superman. The Man of Steel is the prime example of the most morally-pure superhero in the entire genre, perhaps even eclipsing Captain America himself, and is easily the most famous superhero in the world. While other DC heroes like Wonder Woman have used the hammer only (seen in 1996's Marvel vs. DC), only Superman has held both hammer and shield during the single most important crossover event between the two companies.
In 2004's JLA/Avengers crossover, the two sides did not simply fight to preserve one universe over another as was the case with the 90s event - they fought over ideals (the Avengers thought the Justice League was adored by citizens because of Draconian policies, and the Justice League loathed the Avengers for not doing enough to protect the world). The villain Krona, who wishes to control both universes, seems unstoppable until Superman attacks with both Captain America's shield and Thor's hammer, ending the final fight. It is notable that Thor threw the hammer to Superman; there was no question in the God's mind that Clark was worthy.
The hammer and shield are also important symbols of the scope of the Marvel Universe. The former is a tool made by human science during the 1940s, and the other a weapon crafted by a God for a God. Captain America's shield and Thor's hammer were created in the relative early days of the company, but they've stuck around long enough to be wielded by a select few superheroes - even one from a different company altogether - and will likely remain in the Marvel Universe for decades to come.