Having previously played football for Harrow School, Sumner represented Oxford University, where he was described as "most effective", winning praise for his "fine runs", "vigour" and willingness to work hard for the team. In 1872, the university entered the FA Cup at the first round stage, defeating Crystal Palace 3–2 on 26 October 1872, with Sumner scoring one of the university's three goals. The university won their next three matches to reach the semi-final, where they were drawn against the leading Scottish club, Queen's Park. Queen's Park, however, were unable to raise the funds to travel to London and withdrew from the competition, giving Oxford a bye into the final. In the final, played at Lillie Bridge on 29 March 1873, the university met the defending champions, Wanderers who, under the original rules of the competition, were exempt from the earlier rounds. Sumner played as one of eight forwards for the university, who dominated much of the match, but conceded a goal after 27 minutes, when the Wanderers captain Arthur Kinnaird outpaced the university's backs and kicked the ball between the goalposts. In a desperate attempt to secure an equalising goal, Oxford took the unusual step of dispensing with the use of a goalkeeper and moved Andrew Leach upfield to play as a forward. This plan back-fired at around the 80-minute mark, however, when Charles Wollaston broke through and scored a second goal for the Wanderers, who thereby retained the trophy. After leaving university, Sumner played as an amateur for High Wycombe and represented Buckinghamshire.
Later career
Following his graduation in 1875, Sumner worked as a schoolmaster and in 1881 he was teaching at a preparatory school run by Alfred Hyde Harrison at The Lodge, Dunchurch, near Rugby, Warwickshire. By 1890, he had moved to the Cambridge, Massachusetts in the United States, where he married Abbie Page Carew on 4 June 1890 in Cambridge, Massachusetts. Soon afterward the couple moved to Yampa, Colorado and had two children: Mary Beatrice and John Robert Carew Sumner. In 1900, he was living at Yampa, Colorado where he appears in the Harrow Register as a "rancher". On 16 July 1915, he and Abbie sold their former home at 29 Everett Street, Cambridge, Massachusetts to Edith Lesley, who had established the Lesley University at the property. Sumner died at the Scripps Memorial Hospital in La Jolla, near San Diego in California on 15 October 1933.