Of Swedish descent, Sponberg had a very successful club career, ultimately winning three grand finals with two different clubs.
A Canterbury junior, Sponberg made his debut for Western Suburbs in Round 6 of the 1932 season. He played for Wests for three seasons between 1932 and 1934. Sponberg played in the final against South Sydney after a semi-final win against Balmain. However, as minor premiers, Souths were allowed right of challenge, and so a rematch of the grand final was played a week later. Western Suburbs lost the grand final 12-19 and Souths took their 11th title. He scored his debut try in round 11 of the 1932 season in a win against Sydney University.[3]
He joined the newly admitted Canterbury-Bankstown club for their debut season in 1935. He played a total of 8 seasons with Canterbury between 1935 and 1944, including the 1938 Grand Final and the 1942 Grand Final.[4] He spent the 1940 season as captain-coach of Wollongong, before returning to Canterbury in 1941. In round 4 of 1941, Sponberg scored two tries against South Sydney. In Canterbury's season rematch with Souths, Sponberg also scored two tries to help his team win 16-13.[5]
In 1944, Sponberg played his final career game in round 8 against Newtown. He scored a try in that game.[6]
Representative career
Sponberg represented New South Wales on 7 occasions between 1933 and 1935. He scored a try in New South Wales' 24-0 Game 1 win against Queensland in the 1933 Interstate series. He missed 1936 selection due to broken ribs.[7] Sponberg also represented New South Wales City, making 4 appearances with thffe representative team. In his debut with the team, he scored 2 tries against New South Wales Country, with his team winning 47-6.[8]