The first name of the "Mother" was unveiled in the first season of How I Met Your Mother. The character, whose name was Tracy, was played by actress Cristin Milioti. She first appeared in the HIMYM season 8 finale before she was promoted to a lead role in the series' ninth and final season.
The identity of the Mother was the drive for the CBS sitcom. As Future Ted (Josh Radnor) told his increasingly long story to his ever-exasperated children, more and more details came out about the character prior to the official reveal. After years worth of clues, Milioti was introduced as Tracy, the mother of Ted's two children. Her full name, Tracy McConnell, wasn't disclosed until the How I Met Your Mother series finale.
Before Tracy appeared on the series, her character was indirectly shown in a few scenes. Viewers also learned that Tracy was a member of a band, went to the St. Patrick's Day party where Ted ran into the "Bump Girl," and happened to be the roommate of one of Ted's ex-girlfriends. Despite all of those facts and more, one important detail about the Mother was actually shared in the very first season: her name.
In the How I Met Your Mother season 1 Thanksgiving episode, "Belly Full of Turkey," Ted and Robin (Cobie Smulders) ended up going to a strip club with Barney (Neil Patrick Harris). They went there because the Thanksgiving dinner was "surprisingly good" at the Lusty Leopard according to Barney. When Ted bought a lap dance for a homeless man, one of the strippers complimented him on his act of generosity. She introduced herself as Tracy, which Future Ted proclaimed to his kids as being the true story of how he met their mother. They reacted in shock before he confessed that he was joking - perhaps because they recognized the name as that of their real mother.
Some viewers immediately (and, it turns out, correctly) jumped on the name and concluded that the Mother was undoubtedly named Tracy. Of course, there were plenty of other candidates that viewers put their bets on throughout the series. Victoria was high on that list and was even considered a back-up Mother in case the series was canceled before season 9. How I Met Your Mother waited nearly eight years before mentioning the name again. In that time, the Mother's backstory grew and Milioti was eventually cast in the role.
It's unclear if How I Met Your Mother had always planned to always use Tracy as the Mother's name or this connection was retroactively worked into the final season. It would make sense that showrunners Carter Bays and Craig Thomas planned it so early and hoped viewers forgot by the time the character was officially introduced, although it's never been officially stated. Either way, How I Met Your Mother had a way of keeping viewers on their toes with quick details.