Here's what happened to Mr. Sulu after Star Trek: The Original Series and the six Star Trek movies starring the classic crew of the Starship Enterprise. From Star Trek's premiere in 1966 to the final TOS movie, Star Trek VI: The Undiscovered Country in 1991, George Takei portrayed Hikaru Sulu, the original helmsman of the Enterprise. Takei also reprised his role in a memorable episode of Star Trek: Voyager.
In Star Trek: The Original Series, Lieutenant Sulu was regularly seen on the bridge piloting the U.S.S. Enterprise. As a Command Division Officer, Sulu also took command of the ship when Captain Kirk (William Shatner) and Mr. Spock (Leonard Nimoy) were on away missions, which was often. Sulu served aboard Kirk's Enterprise during their original five-year mission and, after being promoted to Lieutenant Commander, returned as the Enterprise's helmsman during Star Trek: The Motion Picture. From Star Trek II: The Wrath of Khan to Star Trek V: The Final Frontier, Sulu, now a Commander, continued as helmsman of the Enterprise and Enterprise-A. However, as a swashbuckler at heart, Sulu yearned to run his own ship, which finally happened in 2290 when Captain Sulu was given command of the U.S.S. Excelsior. Under Sulu's leadership, the Excelsior completed a three-year mission cataloging gaseous anomalies in the Beta Quadrant before they joined the Enterprise to expose a conspiracy that threatened the Federation-Klingon peace treaty.
After saving the galaxy together one last time in Star Trek VI, which was the final appearance together by the entire TOS crew, Sulu's Excelsior parted ways with Kirk's Enterprise. From then on, comparatively little has been revealed about Sulu's later Starfleet career. At some point in the 2270s, Sulu fathered a daughter, Demora Sulu (Jacqueline Kim), who followed in Hikaru's footsteps and became helmsman of the U.S.S. Enterprise-B. Kirk, Montgomery Scott (James Doohan), and Pavel Chekov (Walter Koenig) met Demora when they were guests aboard the Enterprise-B's maiden voyage, as seen in Star Trek Generations, though Kirk was flabbergasted and wondered aloud, "When did [Sulu] find the time to have a family?"
Unlike Kirk, Spock, Bones McCoy, or even Scotty, Sulu did not appear on Star Trek: The Next Generation or interact with Captain Picard and his crew in a TNG movie; therefore, most of what canonically happened in his later life remains a mystery. Most of what's known about Sulu's career after his final TOS appearance in Star Trek VI is through spoken dialogue and ancillary Star Trek materials. However, the deluxe edition of the 2012 reference book Star Trek: Federation: The First 150 years comes with an audio introduction by Admiral Sulu (voiced by George Takei), which reveals that the legendary officer became Commander-In-Chief of Starfleet Command.
Takei reprised his role as Captain Sulu in a memorable episode of Star Trek: Voyager season 3 titled "Flashback". Much of the episode took place in the memories of Lieutenant Tuvok (Tim Russ), who mind-melded with Captain Janeway (Kate Mulgrew) and shared his experience serving as an Ensign aboard Sulu's U.S.S. Excelsior. However, the events depicted in "Flashback" took place during Star Trek VI at the tail end of Sulu's three-year mission commanding the Excelsior. Janeway did remark that Sulu was honored with a holographic portrait at Starfleet Headquarters, which is still on display in the 24th century.
There's no canonical indication of if and when Hikaru Sulu died or whether he is somehow still alive in the late 24th-century era that will soon be seen in Star Trek: Picard. However, Sulu epitomized the bold, risk-taking 23rd century-era Starfleet Officer and he was a part of numerous pivotal conflicts and events where, along with his fellow Enterprise crew members, he literally saved the galaxy in Star Trek.