Sentry milk-vetch has silvery haired leaves and stems. The flowers have pale, purplish-pink petals and white-tipped wings (Barneby 1964).

Sentry milk-vetch has a cushion-shaped growth form. Its flowers have short, bell-shaped sepals, which are the leaf-like structures behind/underneath the petals when the flower opens. (Barneby 1964). The sentry milk-vetch is known to exhibit (Barneby 1964): 
•    Compact, 3 to 12 millimeter (0.1 to 0.5 inch) long, leaves that bear 5 to 9 tiny leaflets arranged opposite each other (pinnately compound) on the stem.
•    Small flowers with banners (the upper part of the petal) 5 to 6 millimeters long (0.2 inch) and keels (two lower united petals) not over 4.5 millimeters (0.2 inch) long.
•    Pistils with 4 to 6 ovules. 
•    Obliquely egg-shaped and densely hairy pods, 3 to 4.5 millimeters (0.1 to 0.2 inch) in length.

Sentry milk-vetch is a perennial. We do not know the longevity of the plants.

Sentry milk-vetch exhibits two episodes of flowering from March through April and from September through November (Maschinski 1990a). The plant most commonly flowers and successfully produces fruit in the spring (Maschinski 1990a). Seeds produced in the fall may not germinate until the following year (Maschinski 1991). Plants in the wild may reproduce as early as one year from germination in individuals that have not been stressed by external factors (e.g., damage to foliage or lack of moisture) (Warren 1993).

Pollinator studies have documented the mason bees Osmia ribifloris and O. ribifloris ribifloris as the primary spring pollinators of sentry milk-vetch when the plant flowers (Busco 2009, 2011, 2013; Busco and Douglas 2010; Busco and Makarick 2012; and Dowlatshahi et al. 2014). The studies also noted several other small bee species pollinating the plants.

The largest plants typically produce the greatest number of mature fruits and seeds, around 200 fruits (Warren 1993). Smaller-sized individuals produce disproportionately fewer fruits than projected based on the size of the plant (Warren 1993). Fruits contain an average of 3 seeds but can vary from one to six seeds (Maschinski 1990a, 1991).

Seed persistence in a seed bank may be minimal due to the shallow soils, large areas of exposed bedrock and exposure of the site to high winds (Maschinski et al. 1994). Seed dispersal is limited; seedlings often germinate within 10 centimeters (4 inches) of an adult plant (Maschinski et al. 1994). Seedling establishment may occur within the foliage of the parent plant or other species.

Sentry milkvetch is a variety of Astragalus cremnophylax. The two other varieties occur near the range of Sentry milk-vetch but do not co-occur with it (Barneby 1992).

Sentry milk-vetch forms mats or shallow mounds in scarcely visible cracks, in sand-filled hollows of rock, or on shallow gravelly soils on one specific, pure white layer of Kaibab limestone (Barneby 1964). Populations occur where the bedrock forms an unshaded platform adjacent to the canyon rim. In comparison with unoccupied sites along the South Rim of the Grand Canyon, soils at the Maricopa Point population have less lime content, slightly greater magnesium content, mixed mineral gravels present, and less residual soils of limestone origin (Prevost 1991). Soils shallower than 2.5 to 5 centimeters (1 to 2 inches) in depth may not have enough moisture retention for survival of seedlings (Maschinski 1990b).