Located 5 km west of Howick, the reserve consists of Midmar Dam, a large impoundment on the Umgeni River that is surrounded by gently undulating, grass-covered hills and lower-lying open grassland and vleis. The area ranges in altitude from 1 040 to 1 138 m a.s.l. Soils are rocky and shale-based, with shallow topsoil. The dam itself has a steep profile and is fairly barren. The climate is warm-temperate. Rainfall averages 775 mm p.a., falling mostly in summer (October–March). The terrestrial vegetation consists primarily of Southern KwaZulu-Natal Moist Grassland. Some of the 'grassland' is actually old farm plots that have recovered quite well. There is a small scrub forest where the Umgeni River flows into the dam.
The grasslands and vleis support several threatened species, including Grey Crowned Crane Balearica regulorum and Blue Crane Anthropoides paradiseus. The vlei also occasionally holds African Marsh Harrier Circus ranivorus. The surrounding grassland supports Black-rumped Buttonquail Turnix nanus, Broad-tailed Warbler Schoenicola brevirostris, Denham's Bustard Neotis denhami, Black-bellied Bustard Lissotis melanogaster and Black-winged Lapwing Vanellus melanopterus. A pair of Martial Eagles Polemaetus bellicosus also regularly forages in the reserve.
Globally threatened species are Blue Crane (flocks of up to 50 birds) and Grey Crowned Crane (flocks of up to 50 birds), which are also seen in surrounding farmland. Black-rumped Buttonquail is a regionally threatened species.
A population of oribi Ourebia ourebi occurs naturally in the area. Two important small mammals are least dwarf shrew Suncus infinitesimus and African striped weasel Poecilogale albinucha. Two Red Data reptiles are present: Gunther's dwarf burrowing skink Scelotes guntheri and Bourquin's dwarf burrowing skink S. bourquini. The latter is a localised endemic and Midmar is the only reserve in which it occurs.
Early in its history, Midmar Dam attracted lots of ducks and geese, but this was a result of eutrophication, driven by the decay of inundated vegetation. Once this resource had been exhausted, the true character of the dam became apparent and duck numbers have never been high since. The intensive use of much of the water surface for human recreation discourages the congregation of large numbers of ducks. In 2004 the dam wall was raised, resulting in the loss of approximately 54% of the original grassland and wetland habitat. Invasive alien plants such as wattle Acacia dealbata and A. mearnsii, bramble Rubus species and bluegum Eucalyptus species occur along the western, northern and eastern shorelines.
Midmar was proclaimed a nature reserve in 1968 and is administered by EKZNW. It conserves an important component of the Vulnerable Southern KwaZulu-Natal Moist Grassland, and the grasslands of the reserve are some of the last remaining in an area that is heavily transformed or degraded.