Small companies added 82,000 jobs while moderate-sized firms gained 60,000 and large firms 57,000, ADP said.
ADP said 198,000 jobs were added to the economy in June, a revision of an earlier report that put the figure at 188,000.
The firm said the manufacturing sector added 5,000 jobs, financial firms gained 4,000 and construction added 22,000 in July.
Among all categories, professional business services gained the most jobs, adding 49,000 positions, ADP President and Chief Executive Officer Carlos Rodriguez said.
ADP has reported an average gain of close to 167,000 jobs per month over the past 12 months.
July's gain of nearly 200,000 is the highest monthly gain since December 2012 when the private-sector added 209,000 positions.