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Bombus mixtus belongs to the diverse subgenus Pyrobombus Dalla Torre which is characterized by a malar space of medium length but longer than its apical width and antennal flagellum 2.5 to 3x the length of the scape. The penis valves of the males are usually hook shaped (Thorp et al., 1983). Abdominal segment 3 of the large bodied B. mixtus is typically covered in black pile with reddish hair on the apical portion (Curry 1984; Franklin 1912). The remainder of the segments are yellow, cloudy yellow, or reddish yellow (Curry 1984; Franklin 1912). The pleura and the face are primarily yellow (Franklin 1912) and the mesonotum has an extensive amount of black pile between the wing bases but it does not form a distinct band (Curry 1984). There is large colour variation in males and they may look similar to B. edwardsii or B. sitkensis however, B. mixtus males have very distinct hair fringes on the inner faces of the antennal flagellomeres (Thorp et al. 1983). The malar space is as long as it is wide (Thorp et al., 1983). Body size and wingspan varies between castes: queens are 11 to 15 mm with wingspans of 27 to 31 mm, workers range between 7 to 11 mm with wingspans of 17 to 25 mm, and males are 8 to 11 mm with wingspans of 21 to 25 mm. Wings are lightly stained brown (Franklin 1912).

Bombus mixtus has an annual colony cycle. Queens emerge in early April from shallow hibernacula dug into the soil to forage and find suitable nest sites, often in abandoned mouse or bird nests. Pollen is collected and manipulated by the founding queen into a ball. Eggs are laid in vertical rows on the top of the ball and covered over with pollen and wax. A nectar pot previously constructed allows the queen to feed while incubating the brood clump at 30-32° C. Larvae hatch after 4-5 days and begin to feed on the pollen mass. The queen continues foraging and regurgitates nectar to the larvae through openings on the top of the brood cells. After 4 molts, larvae spin loose silk cocoons and pupate. The queen now lays a second and third batch of eggs on top of the pupal cocoons using the pollen and wax from the first batch. Female workers emerge 4-5 weeks after the first eggs are laid and take over foraging and nest construction activities. The queen now exclusively constructs egg cells and lays eggs. As the colony expands upwards and outwards and workers increase in number, fertilized eggs become young queens and males emerge from unfertilized eggs. Caste differences are physiological and large numbers of workers are able to provide the food necessary to rear queens. Males are often produced before the new queens and will leave the colony almost immediately after emergence. Young queens may perform both nest and foraging duties prior to mating. Both sexes mate multiple times. Males will mount the queens in the air and continue coitus for several minutes on a nearby surface until kicked off by the female. Once mated, queens prepare for hibernation by eating and increasing vital fat body reserves. The colony declines in late October; workers, males, and the original queen die. The newly mated queens overwinter in small cells in the soil in preparation for spring. (Adapted from Alford 1975 and Thorp et al., 1983).

Polylectic, adults consume nectar and pollen from a variety of genera such as Ceanothus, Lupinus, and Rhododendron and are associated primarily with 6 different plant families in California (Thorp et al., 1983)

Common enemies include the parasitic cuckoo bumble bee (Psithyrus spp.), which will kill the queen and usurp an established colony. The principle parasite of the nest is the sarcophagid fly Brachicoma (Thorp et al., 1983).