Although Tobusch fishhook cactus was previously observed in streamside gravel bars, as noted by D. Weniger in 1979, these are atypical habitats. Almost all populations occur in upland sites dominated by Ashe juniper-live oak woodlands and savannas on outcrops of Cretaceous limestone. Soils are classified in the Tarrant, Ector, Eckrant and similar series, as noted in the 2017 species assessment. Within a matrix of woodland and savanna, the species occurs in discontinuous patches of very shallow, gravelly soils where bare limestone and rock fragments comprise a large proportion of the surface cover, as K. Sutton and others documented in 1997. Associated vegetation includes small bunch grasses and forbs. The distribution of this species within habitat patches is clumped and tends to be further from woody plant cover. The presence of spikemosses (Selaginella spp.), and perhaps other cryptogams, may be useful indicators of fine-scale habitat suitability, as was noted in the U.S. Fish and Wildlife Service 5-year review in 2010. C.M. Reemts and J.R. Ferrato documented in 2020 that wildfire, including prescribed burning, causes negligible damage to Tobusch fishhook cactus populations. The species probably does not require fire for germination, establishment or reproduction, but periodic burning may be necessary to prevent the encroachment of woody plants into its habitats.

Mature individuals have hemispheric or columnar stems from 1.2 to 3.9 inches (3 to 10 centimeters) tall and 0.8 to 3.9 inches (2 to 10 centimeters) in diameter, as documented in 1995 by R.T. Emmett 1995 and others in more recent years. The stems bear a spiral arrangement of conical tubercles that are about 0.4 inches (1 centimeter) long. The tubercles are grooved on their upper surfaces. From seven to 12 short, straight spines are radially arranged at the tips of tubercles, and three to five longer spines project outward from the centers of the spine clusters; one of the central spines is abruptly curved like a fish hook. The roots spread from a short, thick taproot, and often penetrate crevices between rock strata.

Tobusch fishhook cactus grows slowly, reaching a reproductive size of about 0.8 inches (2 centimeters) in diameter after nine years, as noted by R.T. Emmett in 1995 and later in 2002 by J.M. Poole and G.K. Janssen. The average diameter of 1,103 individuals at The Nature Conservancy’s Love Creek Preserve was 0.7 inches (1.7 centimeters), as documented by C. Reemts in 2014. In the wild, the largest individuals, measuring 1.6 to 2.4 inches (4 to 6 centimeters) in diameter, are about 50 years old, as was documented by R.T. Emmett in 1995.

Tobusch fishhook cactus reproduces primarily through outcrossing between unrelated individuals. Flowering occurs between late January and mid-March, and the major pollinators are honey bees and halictid bees, as R.T. Emmett noted in 1995 and was later noted by several other researchers. The purplish-green fleshy fruits ripen in mid-May and split open when dry. Each fruit produces from 20 to 40 papillate seeds that are 1.5 millimeters long by 1 to 1.5 millimeters wide, as documented by R.T. Emmett in 1995. Some seeds fall in the immediate vicinity of the parent plant and may be dispersed by rainwater. Some seeds are dispersed locally by ants. The seeds have an appendage, called a funiculus, that ants feed on; the seeds probably end up in the ant colony refuse mounds, which may be ideal sites for germination and establishment. The edible fruits may also be eaten by rodents or birds, which would provide a means for longer-distance dissemination, but this has not been confirmed. The longevity of seed viability in the soil is unknown.

J. Poole and others documented in 2007 that the Tobusch fishhook cactus is distinguished by the pale to bright-yellow color of the inner side of its tepals, or the undifferentiated petals and sepals, fewer radial spines and fewer ribs, when compared to its closest relative, short-spined fishhook cactus (S. brevihamatus ssp. brevihamatus). The inner-facing tepals of short-spined fishhook cactus are pink or brown-tinged. Additionally, subspecies tobuschii is endemic to limestone outcrops of the Edwards Plateau, while subspecies brevihamatus occurs in alluvial soils in the Tamaulipan Shrublands and Chihuahuan Desert. In 2015, N. Rayamajhi documented that these genetically divergent subspecies may interbreed in a narrow area where their ranges overlap. Grooved cory cactus (Coryphantha sulcata) is very common in the Edwards Plateau and may at first be mistaken for Tobusch fishhook cactus. Both are hemispherical to short-columnar cactuses with yellow flowers and grooved tubercles. The Coryphantha is easily distinguished, however, because all its spines are straight, none are curved like fish hooks.