Nilsen played for Norwegian club Skiens Grane before moving to the United States in 1923. When he arrived, he settled in Boston where he began playing for local amateur and semi-professional soccer teams including the Norwegian-Americans and Hub F.C., both in the Boston and District League.
Boston
In 1926, Nilsen signed with the Boston Soccer Club of the American Soccer League. At the time, the ASL was establishing itself as one of the top leagues, both nationally and internationally, and the Wonder Workers were a rising team in the league. Nilsen began as a right half, then moved into the front line as either a center forward or right inside. He won his first league cup in 1927, then his first league title in the 1927–28 season. The next year, he tied János Nehadoma of the Brooklyn Wanderers as the top goal scorers with 43 goals a piece. He also joined Viking F.C. for a tour of Scandinavia. Viking F.C. was an ad hoc traveling team composed of U.S.-based players with ethnic roots in Scandinavia.
Fall River
Nilsen began the 1929–30 season in Boston, but was transferred to the Fall River Marksmen after three games. In Fall River, he played alongside Bert Patenaudeon the front line. The onset of the Great Depression in 1929, combined with the costs imposed by the "Soccer Wars" between the ASL and United States Football Association led to severe financial problems with the league and its teams. This led to the rapid collapse of several teams and the eventual collapse of the ASL in 1933. Before these happened, Nilsen would win his first "double" as the Marksmen took the 1929–30 league title and the 1930 National Challenge Cup. With financial problems mounting, Sam Mark, owner of the Marksmen, merged the team with the New York Soccer Club to form the New York Yankees. The Yankees competed in the spring 1931 season. While the league standings reflect the Yankee name, the team won the 1931 National Challenge Cup under the Fall River Marksmen name. As in 1928, Nilsen joined a collection of U.S.-based Scandinavian players in 1931 to make a tour of that region, this time as part of Brooklyn Gjoa.
To make matters more confusing, in summer of 1931 the Yankees moved to New Bedford, Massachusetts, merged with Fall River F.C. and became the second team to be named the New Bedford Whalers. Despite the turmoil, Nilsen and his teammates added another "double" to their resumes as they took both the 1932 league title and the National Challenge Cup. In league play, Nilsen finished second to ex-teammate Patenaude for the scoring title. The Whalers won the 1932 Challenge Cup title over the St. Louis-based Stix, Baer and Fuller F.C.. Both legs of the cup final were held in St. Louis. The first ended in a 3-3 tie. A week later, Whalers won the game and the title with a 5-2 victory; Nilsen scoring one of the goals. After the games, Whaler's Scottish international Alex McNab decided to join Stix as a player/coach for the 1932–33 season. McNab convinced several of his Whalers teammates, including Nilsen, Billy Gonsalves, Bill McPherson and Billy Watson to also make the move west.
St. Louis
The addition of the core of the Whalers team to an already strong Stix, Baer and Fuller team created a team which dominated the St. Louis and Challenge Cup competitions for several years. The team ran away with the 1932–33 St. Louis Major League title, then won the 1933 National Challenge Cup title, giving Nilsen his third "double". Stix, Baer and Fuller won both games, 1-0 and 2-1 over the New York Americans. Nilsen again scored in the second game. Stix, Baer and Fuller repeated the double the next year as it again won the league and open championships, giving Nilsen five consecutive Open Cup titles and four "doubles". Following the 1934 National Challenge Cup title game, St. Louis Central Breweries F.C. took over sponsorship of the team. Nilsen remained with Central Breweries which won the 1934–35 league championship, but did not participate in the team's 1935 Challenge Cup victory due to injury. The team experienced yet another name change, this time to the St. Louis Shamrocks. Shamrocks went to both the 1936 and 1937 Challenge Cup finals, but lost both times. In 1937, Nilsen joined his last team, South Side Radio before retiring at the end of the 1937–38 season. Nilsen was inducted into the St. Louis Old Time Soccer Players Hall of Fame in 1983.
International career
In 1934, Nilsen earned two caps with the United States when he was selected to the 1934 FIFA World Cup roster. His first game with the U.S. came on a May 24, 1934 World Cup qualification victory over Mexico. His next game did not end as successfully for the U.S. as it lost to Italy on May 27, 1934 in the finals first round.
Managerial career
Nilsen coached the St. Louis Raiders during the 1946–47 season.
Personal life
According to the Soccer Hall of Fame, Nilsen worked as a machinist and a men's clothing model in both Boston and St. Louis.
National Soccer Hall of Fame
The National Soccer Hall of Fame inducted Nilsen in 2005 as part of a process of recognizing significant pre-1950s players. According to the Hall of Fame, "We were aware that in the early decades of the Hall of Fame a number of outstanding players had slipped through the cracks of the selection process. In order to correct these oversights we established a Blue Ribbon panelconsisting of historians Colin Jose, Roger Allaway and Hall of FamerWalter Bahr, to review the credentials of all Veterans from the pre-NASL era. Out of a total of 150 players who met the eligibility criteria, the panel unanimously recommended, and the Board approved, the special induction of these five players." Nilsen and Alex McNab, teammates on both the Whalers and Stix, Baer and Fuller were among the five selected.
