The short answer is at his original firm in New York. However, he reaches the equivalent or higher rank elsewhere. Mike's Rank at Pearson Specter Litt
Mike becoming Senior Partner was the final piece of character development the show needed to let him exit. It showed that he had evolved from the guy sneaking in the back window to the guy owning the building. does mike ross make senior partner
So, does Mike Ross make Senior Partner?
After the Season 5 finale, Mike makes a deal to keep Harvey out of prison and serves two years in federal prison for fraud. Upon his release, he is barred from practicing law in New York for a time. The short answer is at his original firm in New York
While Mike never achieved "Senior Partner" status at the firm where he started (variously known as Pearson Hardman, Pearson Specter Litt, etc.), he did break records during his tenure: It showed that he had evolved from the
While he eventually leaves the firm later in the series to move to Seattle with Rachel (working at a legal clinic again), the moment he was offered the Senior Partnership card was the moment he finally "arrived."
As the series progresses, Mike's exceptional abilities and his unique situation (being a "legal genius" without a law degree) lead to significant contributions to the firm, Pearson Hardman, later renamed Pearson Specter Litt, and eventually, Litt Wheeler. His progression through the ranks and his desire to become a senior partner are significant plot points.