Barclay was 25 years old when he broke into the big leagues on April 17, 1902, with the St. Louis Cardinals. Two of the team's starting outfielders, Jesse Burkett and Emmet Heidrick, had jumped to the rival St. Louis Browns of the new American League, and Barclay, along with Doc Smoot, were acquired to replace them, with Barclay becoming the everyday left fielder. Both men hit over .300 that year, as did their manager and outfield mate Patsy Donovan, but the Cardinals still fell from fourth to sixth.
In 1903, although Smoot continued to play well, Barclay's production fell off, batting just .248. Still, he was brought back to be the team's starting left fielder in 1904. His performance continued to decline, and he was batting just .200 when he was sold to the Boston Beaneaters on September 11, 1904, having been replaced by rookie Hugh Hill.
Barclay was moved to right field by the Beaneaters, and his average bounced back a bit to .226 during his time with Boston. He started the 1905 season back in left field, but after batting just .176 in 29 games he was replaced by Jim Delahanty and eventually released on May 22, 1905.
Barclay returned to Rochester to finish the 1905 season, but he never regained his previous level of performance, as he batted .245 that year, then just .190 in 1906. After one more season with the class-B Lynn Shoemakers of the New England League, batting .207 in 43 games, Barclay left professional baseball.
Barclay was credited with inventing the first-ever football helmet in 1894, with the intention that it would prevent cauliflower ears. The helmet was constructed by a saddle-maker from nearby Easton with strips of leather harness padding. It attracted national attention in a Lafayette game against Penn on Oct. 24, 1896. The NCAA and the National Football League made helmets mandatory in 1939 and in 1941, respectively.