The firm booked a 54% rise in pre-tax profit to £18.6 million in the six months to June 30, while revenue jumped nearly 44%% to £1.19 billion.
Total used car unit sales grew 40%, or 5.8% on a like-for-like basis.
Boss Daksh Gupta said the numbers represented a “period of record trading”, adding that the group is “well positioned to execute its growth strategy moving forward”.
The performance was boosted by acquisition of Ridgeway Garages, which Marshall Motors acquired last year.
Mr Gupta said: “In the two years since listing, the group has successfully completed a number of retail acquisitions transforming its scale, geographic footprint and franchise portfolio as well as significantly growing its profitability.”
However, the increase in used car sales comes amid a fall in comparable new vehicle sales.
Marshall Motors said new car retail unit like-for-like sales fell 0.4%, reflecting figures from the Society of Motor Manufacturers and Traders (SMMT) that Brexit uncertainty is putting the brakes on the market.
Figures from the trade body in August showed the car market has declined for a fourth consecutive month.
Marshall Motors said: “The board is cognisant of the economic and political uncertainty following the UK referendum on EU membership and industry forecasts for continuing declines in the UK new car market.
“The board therefore remains cautious.”