Bill Mikkelson joined the Brandon Wheat Kings junior hockey club in 1964-65, beginning a four-year association with the club team. In his final season he scored 14 goals and 41 points in 56 games in WCJHL. He spent the next two years patrolling the defense for the Winnipeg Jets in the same league.
In September, 1970 Mikkelson was signed as a free agent by the NHL's Los Angeles Kings. He played the entire 1970-71 season in Springfield of the American League, scoring eleven points in 69 games. In 1971-72 he got his first taste of NHL experience, dressing for 15 games with the Kings, picking up one assist.
The expansion New York Islanders claimed Mikkelson from the Kings in the June, 1972 Expansion Draft. During the Islanders inaugural season Mikkelson played his only complete NHL season, dressing for 72 games, scoring a goal and eleven points. After spending the entire following season in Baltimore of the AHL, Mikkelson was again claimed in the Expansion Draft on June 12, 1974 when the Washington Capitals selected him. He suited up for 59 games with the Caps that year, scoring three goals and ten points. Once again he spent an entire year in Baltimore and played just one more game in the NHL with the Capitals in the 1976-77 season. The rest of his year was filled playing with the Hershey Bears and the Rhode Island Reds. He retired at the end of the season at the age of 29.
Mikkelson appeared in 147 NHL games, scoring four goals and 22 points.