Vedantic thought overlays the five koshas with a second, coarser scheme of three bodies, or sharira: the gross, sthula sharira, the subtle, sukshma sharira, and the causal, karana sharira. Manomaya, the mental sheath, belongs to the subtle body along with the vital and wisdom sheaths, set apart from Annamaya, the food sheath, which alone is identified with the gross body and left behind at death.
Because Manomaya sits in the subtle rather than the gross body, the tradition holds it to persist beyond physical death, carrying the ordinary restless mind, its habits, doubts and reactions, forward until liberation dissolves the subtle and causal bodies as well; this is part of why Vedantic teaching treats disciplined control of Manomaya, quieting its restlessness, as necessary long before death, not only at the end of life.
Manomaya's restlessness is also what distinguishes it from Vijnanamaya, the wisdom sheath just within it: where Manomaya reacts automatically to sensory input, Vijnanamaya is held to discriminate and judge, and much of Vedantic practice is aimed at strengthening that discriminating sheath's hold over the more reactive mind beneath it. Twentieth century Theosophical writers later borrowed kosha terminology for a separately bounded scheme of subtle bodies, a distinct later system rather than a restatement of this teaching.
Sources The Principal Upanishads with Rainbow Body: A History of the Western Chakra System from Blavatsky to BrennanKurt Leland