At the lake, bird watchers can observe many water birds. The year 2001 marked the first time in recent years that great blue herons were known to nest by the lake. Other water birds include white pelicans, western grebes, cormorants, gulls, Canadian geese and mallard ducks. Another lake dweller is the beaver.

One of the park's most famous residents uses both the lake and the prairie around it. For several years, Standley Lake has been home to a pair of nesting bald eagles. During the summer, the bald eagles can fish in the lake. During the winter, when the lake freezes over, the bald eagles can steal prairie dogs caught by other birds of prey. Some of the other birds of prey, or raptors, seen at the park include Swainson's hawks, prairie falcons and red-tailed hawks. There are also nesting great horned owls, which are birds of prey of the night.

In the 2,000 acres of open space around the lake, one of the most obvious animals is the prairie dog. Prairie dog towns provide food and shelter for more than 100 other species of animals. Many animals in the park, including the bald eagles, coyotes, red foxes and bull snakes, eat prairie dogs. Burrowing owls and rabbits live in prairie dog burrows. Other mammals seen in the park include mule deer and rock squirrels.

Standley Lake Regional Park is a popular destination for fishing and is host to a variety of fish species including walleye, bass, and trout. The lake is stocked regularly throughout the summer by Colorado Parks and Wildlife. In addition, the lake supports a variety of other fish. During the summer months the fishing pier is the best fishing spot, but fishing can be done anywhere on the shore except in areas posted otherwise.

As part of the City of Westminster's Aquatic Nuisance Species Protection Program, aquatic bait and night fishing is prohibited at Standley Lake Regional Park. Aquatic bait includes minnows, leeches, frogs, crayfish, salamanders, etc. The cleaning of fish is allowed only at the fish cleaning station.

Areas permanently closed to fishing include the dam, all boat docks and all boat ramps. In addition, fishing is not permitted within 150 feet of boat ramps and boat docks, with the exception of the Special Needs floating fishing pier.