The Markit Stanbic Bank Kenya Purchasing Managers' Index (PMI) for manufacturing and services fell to 51.0 from 51.2 in February. Any reading above 50 indicates growth.
"The drop in the PMI doesn't really come as a surprise as agricultural productivity is usually weaker in the first quarter," said Jibran Qureishi, regional economist for East Africa at Stanbic Bank.
"As the long rains commence probably from April, higher output from the agriculture sub-sector is likely to underpin private sector activity."
Output by firms, their new orders and employment recorded weaker expansions during the period, while input costs rose at a softer pace, the survey found.
The central bank expects the economy to expand by 6.3 percent this year.
Detailed PMI data are only available under licence from IHS Markit and customers need to apply for a licence.