Joseph Cassidy was born in Dalziel, Scotland, on 30th July, 1872. He played for Motherwell Athletic before joining in 1892. He had been signed to help save the side from relegation from the First Division of the Football League. This had the desired effect as the club beat Small Heath over two legs. After playing only four games he returned to Scotland where he played for Celtic in the Scottish League.
In 1894 Joseph Cassidy returned to Newton Heath who were now playing in the Second Division. In his first home against Walsall Town he scored four goals in the club's 14-0 victory. However, after the game Walsall protested about the state of the pitch and the Football Association ordered a replay. They won that game 9-0. That season Cassidy scored 8 goals in 8 games.
In the 1895-96 season Newton Heath finished in 6th place in the Second Division. Cassidy was top scorer with 16 goals in 19 games. The following season he did even better with 17 goals in the league. He also got 6 goals in Newton Heath's good run in the FA Cup. Except for the 1897-98 season he finished as top goalscorer on five successive seasons.
In April 1900 Cassidy was sold to Manchester City for £250. The club directors admitted that he was the best forward they had ever had but because of Newton Heath's serious financial problems he had to be sold. During his time at the club he scored 100 goals in 174 appearances.
Joseph Cassidy also did well in his first season at his new club ending up as top scorer with 14 goals in 30 games. However, he was sold to Middlesbrough at the end of the season for £75 on the grounds he was not worth his £4 a week wages. The manager of Manchester City, Sam Ormerod, complained about this decision but it was now clear that the directors of the club had lost confidence in him and he was no longer making the key decisions.
Cassidy became a strong favourite with the Middlesbrough fans and over the next five years scored 34 goals in 135 appearances. Cassidy became player-coach at Workington in 1906.
In April 1916 the Manchester Football News reported that "Joe Cassidy, whose connections with Manchester football extended over such a long period, has had a mental breakdown." It is not known whether this was connected to the First World War.