He bowled medium pace and could swing the ball both ways, whilst when wickets were affected by rain he was able to get on a good deal of off-break.
John Arlott said of him, "he bowled more overs, conceded more runs, took more wickets, scored the fastest 50, made more ducks and was not out more often than anyone else in the county's history".
He found he had very little opportunity because Sussex had so many medium-pace bowlers of similar type on their professional staff, notably Maurice Tate and the Relf brothers.
He started slowly, but by 1925 – when Glamorgan's utterly abysmal batting caused them to suffer a record number of defeats in the Championship – he was a well-established bowler and took over 100 wickets for the first time.
Mercer then went on tours of India and Ceylon without doing anything spectacular, and in 1927 when pitches were almost always soft and wet (often so much so as to be really easy for batting) he did not do as well as expected until late in the season.