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This small (25-35 mm wingspan) hairstreak may or may not have tails (Opler et al.; 1995). The colouration of the wings is similar to other hairstreaks in the province, particularly Satyrium acadicum: dorsal wings are grey-brown or reddish with blue sheen and females have a yellowish tinge. The ventral wings are lighter brown and may even be pale grey to white. There is an orange submarginal patch on the ventral hindwing adjacent to the blue spot. However, the extent of the orange areas is smaller in S. sylvinum than in other hairstreaks. There is no orange submarginal spot on the ventral forewing and no orange cap on the blue spot on the ventral hindwing. There is also a postmarginal band of small black dots; some individuals may have only a few spots. Eggs are not described. Larvae are pale green they also have a thin dorsal line that is dark green or white (Layberry et al.; 1998; Opler & Wright; 1999). Pupae are dark brown (Guppy & Shepard, 2001).

Unknown in Alberta. Eggs overwinter in British Columbia (Guppy & Shepard, 2001). Larvae complete development in the spring and feed on willow leaves (Guppy & Shepard, 2001). Males perch on low vegetation near host plants to watch for females (Opler et al., 1995). Eggs are laid singly on stems of host plants (Opler et al., 1995).

Unknown for Alberta, although willow (Salix spp.) have been identified as host plants in the neighbouring province: British Columbia (Guppy & Shepard, 2001). Adults feed on flower nectar (Opler et al., 1995): milkweeds in the United States (Opler & Wright, 1999) and Kondla (2001) documented a nectar feeding event on spreading dogbane (Apocynum androsaemifolium) in Alberta.