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. His second competitive first team appearance was not until 18 months later in September 1978. Peter Withe had been sold to Newcastle and Steve Elliott failed to make the grade. This allowed Birtles a chance that he grasped. He scored his first goal for the club in his third match, a first round 1978–79 European Cup tie against holders Liverpool. Birtles kept his place for the rest of the season capping his success with two goals in the 3–2 win over Southampton in the 1979 League Cup Final. He also won a 1979 European Cupwinner's medal via the 1–0 victory over Malmö FF of Sweden in Munich's Olympic Stadium. He scored 14 times in the league alone that season. He gained another winner's medal in the following season's European Cup against Hamburg providing the return pass from which John Robertson scored the only goal. Birtles also played in every First Division game for Forest that season scoring 12 goals. He began the 1980–81 season in fine form, scoring six goals in nine league games for Forest, before a £1.25 million fee took him to Manchester United.
Manchester United
Under manager Dave Sexton, Birtles made his debut for Manchester United in a First Division victory over Stoke City at the Victoria Ground on 22 October 1980. He made 28 appearances in his first season with the club. 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.
Return to 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.
He spent 18 months with Notts County, during which he appeared in almost every league game. Birtles ended his career with Grimsby Town. They won two successive promotions during his time there, and by the end of his career he had accumulated more than 400 professional league appearances.
International career
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.
