Trumpeter swans establish life-long mates at approximately 3 years of age and nest the following year. Sometime between late March and early May, they build their nests, choosing locations close to the water, either on shore, small islands, or muskrat and beaver lodges. The male (called a cob) gathers nest material, uprooting marsh plants such as cattails, sedges, bulrushes, and horsetail, and brings them to the female (called a pen) for placement. The nest mound, which takes about 2 weeks to build, reaches a diameter of 6 to 12 feet and an average height of 18 inches. The same nest site may be used for several years.

Trumpeter swans grow rapidly. By 8 to 10 weeks of age, young trumpeters have reached half their adult size and are fully feathered. They retain their gray juvenile plumage until the second winter.