He signed with the Chicago Cubs in 1932, and by his second pro season, he had reached the top level of the minors with the PCL Angels, leading the league at age 19 with 43 home runs.

Then came his superlative 1935 season, in which he played in 170 games, scored 157 runs, made 232 hits, drove home 147 RBI, and batted .361.

His 56 homers led the Coast League, but Lillard was surpassed in several other key batting categories by the loop's premier player, Joe DiMaggio of the San Francisco Seals.

Upon returning to the minors in 1937, Lillard converted to part-time pitcher, and he came back to the Majors as a moundsman for the 1939 Cubs and the 1940 St. Louis Cardinals.

Lillard went back to the minor leagues for good in the middle of the 1940 season, and played as a combination pitcher and position player through 1954, with the 1943–1945 campaigns missed during World War II.