He was a long term member of the Stoke Newington Vestry and the Hackney District Board of Works. From 1868 he was the district board's representative on the Metropolitan Board of Works, and when the Metropolitan Board was abolished in 1889 he stood for election for the successor London County Council at Hackney North, but was not elected.
Runtz's biggest contribution to London was his work to create public open spaces. Along with Joseph Beck, a member of the Common Council of the City of London, he was largely responsible for the opening of Clissold Park and Hackney Downs. The two lakes in Clissold Park were named Runtzmere Lake and Beckmere Lake in honour of the two proponents of the purchase of the park.
He died at his home, Linton Lodge, Lordship Road, aged 72, and was buried at New Southgate Cemetery.