The Blue Stockings' successful season in the Northwestern League prompted the team to transfer to the American Association, a major league organization, in 1884. Walker's first appearance as a major league ball player was an away game against the Louisville Eclipse on May 1, 1884; he went hitless in three at-bats and committed four errors in a 5–1 loss. Throughout the 1884 season, Walker regularly caught for ace pitcher Tony Mullane. Mullane, who described the rookie ball player as “the best catcher I ever worked with”, purposefully threw pitches that were not signaled just to cross up the catcher. Walker's year was plagued with injuries, limiting him to just 42 games in a 104-game season. For the season, he had a .263 BA, which was top three in the league, but Toledo finished eighth in the pennant race. The team was also troubled by numerous injuries: circumstances led to Walker's brother, Weldy, joining the Blue Stockings for six games in the outfield.