Mills and his older brother Arthur began their careers playing for Bethanga in the 1926 and 1927 Kiewa & District Football Association grand finals.[1][2][3][4] The Mills family then moved to Oxley, near Wangaratta, and the pair played with nearby Wangaratta Football Club in 1929.[5][6] In 1930, both brothers made their debuts for the Hawthorn Football Club. Bert Mills won Hawthorn's 'most consistent player' award in his first season of VFL football.[7]
Mills usually played as a ruckman, but was also used at centre half-back. He captained Hawthorn at various times during his career, beginning in 1932, and then in single seasons during 1934 and 1938 before his final stint from 1940 until 1941 as captain-coach.[8][9]
During his time at Hawthorn, Mills won their best and fairest award three times, in 1933, 1935 and 1939.[10][11] Mills was also presented with a life membership medallion in 1939.[12][8] He represented Victoria in interstate football, doing so on 11 occasions.[8] In 1936, Mills won Hawthorn's 'most popular player' award, which was sponsored by the Hoyts Palace Theatre, Glenferrie.[13]