Riot Blockchain (RIOT) had the lowest return between 2010 and 2019 by a mid-cap US stock, returning -99.7%.

ASSET	YEARS	% RETURN
Riot Blockchain (RIOT)	2010-2019	-99.72%
Centrus Energy (LEU)	2010-2019	-99.36%
Millicom International Cellular (TIGO)	2010-2019	-99.14%
Civitas Resources (CIVI)	2010-2019	-98.65%
Tidewater (TDW)	2010-2019	-98.55%
Quantum Computing (QUBT)	2010-2019	-98.42%
American Superconductor (AMSC)	2010-2019	-98.11%
Comstock Resources (CRK)	2010-2019	-95.81%
Antero Resources (AR)	2010-2019	-94.53%
Cogent Biosciences (COGT)	2010-2019	-93.87%
TG Therapeutics (TGTX)	2010-2019	-93.69%
Applied Digital (APLD)	2010-2019	-93.44%
Skeena Resources (SKE)	2010-2019	-92.64%
BlackBerry (BB)	2010-2019	-90.58%
Transocean (RIG)	2010-2019	-89.77%
Immunitybio (IBRX)	2010-2019	-89.76%
Eldorado Gold (EGO)	2010-2019	-88.7%
Soleno Therapeutics (SLNO)	2010-2019	-88.47%
Frontline (FRO)	2010-2019	-88.24%
Perpetua Resources (PPTA)	2010-2019	-86.12%
Energy Fuels (UUUU)	2010-2019	-84.73%
Ovintiv (OVV)	2010-2019	-82.65%
Northern Oil & Gas (NOG)	2010-2019	-80.34%
Innodata (INOD)	2010-2019	-79.61%
Cleveland-Cliffs (CLF)	2010-2019	-79.12%

Mid-cap stocks are treated as having a market cap between $2B and $10B.