The humanitarian campaigner claimed on Twitter that the cost was the equivalent of £1,000 for every UK taxpayer.
Defence think tank, the Royal United Services, has revealed £19.59bn had been spent on the Afghanistan campaign between 2006 and 2013 and £9.56bn in Iraq between 2003 and 2009.
Ms Lennox wrote: "It would have funded the employment of at least 1.464.000 [million] new nurses or 408,000 NHS consultants. Invested in education, it would have built 5,250 new free schools.
"The report concludes that policy makers on both sides of the Atlantic fundamentally misjudged the ability of military might and money to affect change in countries with no history of Western-style democracy."