The 18-hole golf course originally opened in 1960 on Riverside Drive between Vineyard and Archibald avenues.
The public was recently given a chance to see the changes in an open house.
"The customers are very pleased, and we've had nothing but rave reviews," said Brenda Hickman, Whispering Lakes general manager.
The golf course was never shut down, but staff members were moved to a small, temporary trailer for a few years while the work was undertaken, she said.
The improvements include a new clubhouse, golf carts, renovated landscape and new equipment.
The new clubhouse offers indoor and outdoor seating for up to 60 guests and an adjoining lawn to host tournament dinners, she said.
The addition also means that patrons can stop by and grab fresh-made sandwiches before or after golf, she added.
But the property wasn't always meant to house a golf course. In 1914, Ontario purchased 175 acres for under $40,000 to use as a sewer farm, said Mark Chase, community and public services director for Ontario.
Through the years the city acquired more and more land and by 1959 Ontario had 242 acres. A contract was negotiated with Ontario Golf Inc to run the facility.
The following year it was renamed Whispering Lakes Ontario Public Course.
Soon the golf course was overshadowed by its snack room, the Royal Tahitian, which garnered national attention.
Performers such as Duke Ellington, Dorothy Dandridge, and George Shearing were part of the Garden Concert series. In 1967, The Four Tops, James Brown, Bobby Darin, the Platters and Ray Charles performed in Ontario.
Due to deteriorating conditions, the clubhouse was torn down in 2010. A temporary clubhouse was moved to a temporary location while work began on the new facility.
"We will continue to make Whispering Lakes Golf Course a place for all of Ontario's residents and the region's visitors to enjoy," Chase said.