Birtles was signed by Nottingham Forest from non-league Long Eaton United for £2,000. He made his Forest debut as a winger in March 1977 in the old Second Division, against Hull City. From that point on he disappeared from first team contention, not making his second appearance until September 1978. Peter Withe had been sold to Newcastle and "prodigy" Steve Elliott failed to make the grade, allowing Birtles an unexpected chance which he grasped, scoring his first goal for the club in his third match, in the European Cup against Liverpool. Birtles kept his place for the rest of the season, capping his success with a fantastic display in the 3–2 win over Southampton in the 1979 League Cup Final (scoring 2 goals) and a 1979 European Cup winner's medal via the 1–0 victory over Malmö FF of Sweden in Munich's Olympic Stadium. He was quickly established as one of the top strikers in England, having scored 14 times in the league alone that season.
He scored his first league goal for United in the 1981–82 season, and went on to score 11 times for them that season.
Coincidentally, his Manchester United spell ended where it began—against Stoke City. His final appearance for them came on the last day of the season, when they beat the Potters 2–0 at Old Trafford. He began the 1982–83 season still with Manchester United, but was not selected for a first team game, and returned to play for Nottingham Forest.
His second spell at Forest included a spell at centre-half. However, he still proved himself to be a competent goalscorer, particularly with 15 league goals in the 1983–84 season and 14 in the 1986–87 season, the last of which he was joint top goalscorer alongside midfield star Neil Webb and up and coming striker Nigel Clough.
However, Forest manager Brian Clough allowed Birtles to part from the club on a free transfer in June 1987.
After an 18-month period with Notts County (for which he appeared in almost every league game), Birtles ended his career with Grimsby Town, helping them to two successive promotions up until 1991, by which time he had accumulated more than 400 professional league appearances.
During the final months of his first spell at Nottingham Forest, when he was still reputed as one of the English league's finest strikers, he was capped three times at senior level for England. His first cap came on 13 May 1980 in a 3-1 friendly win over Argentina. His last came on 15 October that year in a 2-1 defeat against Romania in a 1982 World Cup qualifier.