Mercer played football in the streets of Ellesmere Port: "Back alley football is only a substitute for the real thing. We always have to right for bigger and better playing fields for children. But, all the same, the back alleys did hold some valuable lessons of their own. For instance, playing with a small ball. If you could control a small hall with certainty, you found later that bringing down a normal ball came more easily. It was wonderful training for the eye."
Mercer and his friend, Stan Cullis, both played football forCambridge Road School. Both showed outstanding ability and were selected to play for Ellesmore Port Boys against Chester Boys in January 1929. Two years later Mercer signed for Everton.
In December, 1936, Everton signed Tommy Lawton for a fee of £6,500. It was a record fee for a teenager. One of the attractions of the deal was that Lawton now had the opportunity to play with Dixie Dean. When they met for the first-time, Dean put his arm round Lawton and said: "I know you've come here to take my place. Anything I can do to help you I will. I promise, anything at all." Dean was thirty years old and after suffering several serious injuries, he knew that there was not much time left for him at the top. Dean kept his promise and spent a lot of time with Lawton on the training field.
Six weeks after joining the club, Tommy Lawton was brought into the first team for an away match against Wolverhampton Wanderers, as Dixie Dean was rested prior to a fifth round FA Cup tie with Tottenham Hotspur. Lawton found it difficult playing against the England centre-half, Stan Cullis, however, he did score a goal 15 minutes from the end.
Everton drew the FA Cup tie with Tottenham Hotspur 1-1 and it was decided to play Tommy Lawton alongside Dixie Dean in the replay. In the second minute Lawton scored with a tremendous shot from outside the penalty area. Dean turned to Mercer and said: "Well, that's it then. That's the swan song. That's the end of it." Dean realised that it would not be long before this talented player took his place in the side.
At the beginning of the 1937-38 season Tommy Lawton played at inside-right and Dixie Dean at centre-forward. The pairing did not work and Everton failed to win a game when they two men played together. On 8th September 1937, Dean was dropped and Lawton replaced him as centre-forward to play against Manchester City. Lawton scored in the 4-1 victory and Dean only played in two more games for Everton before leaving the club.
Everton finished in 14th place in the 1937-38 season with Tommy Lawton ending up as the club's top marksman with 28 goals in 39 league games. He was also the top scorer in the whole of the First Division. This was an amazing achievement for someone who was only 18 years old.
Mercer, who was only 24 years old, had developed into a great player for Everton. Tony Matthews in Who's Who of Everton points out that Mercer "was an outstanding wing-half with a biting tackle and a never-say-die attitude." Mercer won his first international cap for England against Northern Ireland on 16th November 1938. The England team that day also included Eddie Hapgood, Stan Cullis, Willie Hall, Tommy Lawton and Stanley Matthews. England won the game 7-0.
Joe Mercer won his fifth and last international cap for England against Romania on 16th November 1938. The England team that day also included Stan Cullis, Tommy Lawton, Frank Broome, Wilf Copping, Len Goulden and George Male. England won the game 2-0.
Tommy Lawton later commented: "Joe Mercer was a tremendous tackler, timing his interventions perfectly, and he loved nothing more than to bring the ball up and take part in an attack with his forwards. He had a deceptive speed, but then Joe was a deceptive player, probably due to those bandy legs of his."
The outbreak of the Second World War brought a halt to the Football League. The British Army invited some of the best footballers to became Physical Training instructors at Aldershot. This included Joe Mercer. Other top players at Aldershot included Wally Boyes, Cliff Britton, Matt Busby, Don Welsh, Billy Cook, Arthur Cunliffe, Archie Macaulay, Norman Corbett, Tommy Lawton, Bert Sproston and Eric Stephenson.
Mercer returned to Everton after the war and stayed at the club until being transferred to Arsenal for a fee of £8,000 in November 1946. It was an excellent signing. According to Jeff Harris, the author of Arsenal Who's Who (1995): "Joe Mercer was one of the most influential players Arsenal have ever had." At the time the club was at the bottom of the First Division.
Mercer was appointed captain of the side and led Arsenal to the 1947-48 league championship and the 1950 FA Cup victory over Liverpool. That year he was voted by the Football Writers' Association (FWA) as their Footballer of the Year Award. Mercer won his third championship medal when Arsenal finished the 1952-53 season beating Preston North End on goal average.
Len Shackleton argued he was convinced that "Joe Mercer, as an Arsenal player between his thirtieth and fortieth birthdays, was a far better wing-half than in his younger days as an Everton and England player. When Joe Mercer discovered his legs would not allow him to run all over the field, when his physical condition hampered him, when he played almost entirely in his own half of the field, he was great. The service of wonderful passes that flowed from this spindly bow-legged genius was, I am certain, fifty per cent of the reason for Arsenal's post-war successes."
On 10th April 1954 Mercer played in a league game again Liverpool. Tommy Lawton, who had recently joined Arsenal, later described how he heard the "sickening crack" when Mercer collided with Joe Wade. His right-leg was fractured in two places and he was forced to retire from playing football.
Mercer ran a grocery shop in the Wirral before becoming manager of Sheffield United in August 1955. He moved to Aston Villa and in 1960 helped the club win the Second Division title. He resigned in July 1964 and the following year he joined Manchester City. He appointed Malcolm Allison as his assistant in 1966 and over the next few years he had great success winning the Second Division (1967), First Division (1968), the FA Cup (1969), League Cup (1970) and European Cup-Winners Cup (1971). In 1974 Mercer was the temporary manager of the England national side.
Joe Mercer died in Liverpool in August 1990.