The club spent almost ten years on a sports field in Wattstraße before moving in 1920 to the Sadowa sports ground, today’s Stadion An der Alten Försterei in the Wuhlheide. For the official opening on 7 August 1920, reigning German champions 1. FC Nürnberg came to visit. In 1920 Union became Berlin champions for the first time and were allowed to take part in the finals of the German championship, reaching the quarter-finals before losing 3–2 to Vereinigte Breslauer Sportfreunde.\n\nIn 1923 the club became Berlin champions again and advanced all the way to the national final after beating Arminia Bielefeld and SpVgg Fürth.
At Berlin’s Grunewaldstadion, Union lost that “home” final 3–0 to Hamburger SV in front of 64,000 spectators. After that, the early golden phase of Union-Ob., as the club was then known, came to an end.