The index grew for the 88th consecutive period, data revealed on Wednesday. Economist polled by Thomson Reuters had forecast the index to only reach 55.8, but a jump in new orders boosted growth higher.
The non-manufacturing index hit 55.2 in March, down from 57.6 in February. This represented continued growth in the non-manufacturing sector but at a slower pace.
A reading above 50 on this index indicates expansion in the service sector, and a reading below 50 indicates contraction.
During April, 16 non-manufacturing industries reported growth, including wholesale trade, utilities, arts, mining and retail trade, the group said.
A survey respondent who works in construction noted that business levels have been increasing, and "more project inquiries are being received."
Overall for April respondents comments were "mostly positive about business conditions and the ... economy," the ISM said.
The ISM's Prices Index increased 4.1 percentage points in April, to 57.6 percent, indicating prices in the sector grew for the 13th consecutive month and at a faster rate.
The Business Activity Index also rose to 62.4 from 58.9 the month before, and the New Orders Index climbed to 63.2, the strongest since August 2005, from 58.9 in March, the ISM said.