Last month, the Federal Reserve launched an aggressive new plan to bolster the economy, saying it will buy $40 billion in mortgage-backed securities per month until the outlook for the job market substantially improves and as long as inflation remains contained.
ISM's forward-looking new orders index jumped to 57.7 in September from 53.7 in August, but growth in employment eased to 51.1 from 53.8. Exports also slowed to 50.5 from 52.
Services firms - which range from agriculture to real estate companies - avoided the contraction that hit their manufacturing counterparts over the summer.
The resilience in the U.S. contrasted with data from abroad, which showed a slowdown in the euro zone service sector worsened last month, and China's services sector expansion slowed to nearly a two-year low.
Trading was choppy on Wall Street following the data, with U.S. stocks advancing modestly by midday. The dollar hit a fresh two-week high against the yen, and Treasuries prices weakened.
Separate data showed demand for mortgage refinancing surged more than 19 percent last week as interest rates tumbled to new record lows in the wake of the Fed's announcement of its latest stimulus plan, referred to by market watchers as "QE3."
More people were also looking to buy a home, with purchase applications rising nearly 4.0 percent.
"While we still think that QE3 will have only a small impact on the wider economy, this demonstrates that monetary policy isn't completely ineffective," said Ashworth.