The Sussexes topped the regal gas-guzzling league thanks to their £245k visit to South Africa last year - which had almost the same carbon footprint an aversive Brit puts out in a year.
Prince Charles, 71, was on the other end of the spectrum thanks to his regular speeches and social media posts highlighting the importance of the environment.
That is despite him having the worst travel footprint of the 10 royals ranked.
The Save on Energy rankings put Camilla, the Duchess of Cornwall, in second place whilst Prince Philip snatched third thanks to his charitable work.
The firm worked out which Royals are the most and least eco-friendly by analysing data on their travel carbon footprint, environmental patronages, environmental engages and eco-conscious lifestyle choices.
A spokesman said: “Despite having been awarded points for owning an electric car and Meghan’s loyalty to sustainable fashion brands, Harry and Meghan engaged considerably less than others with environmental charities and engagements between March 2019 and 2020, which hinders their score.
"They also had the third worst carbon footprint of all royals studied."
The study added: “The couple’s royal tour of South Africa last year had a high cost not only financially (it was the most expensive royal journey of 2019-2020), but in carbon footprint.
"In a single trip, Harry and Meghan emitted almost the same amount of CO2 than an average British person does in an entire year, deducting them 9 points.”
Meanwhile, Kate Middleton was ranked fourth overall while Prince William was placed sixth - with the Duke of Cambridge just this week releasing an environmental documentary.
The firm found that Kate Middleton adopted a green lifestyle through sustainable fashion choices and using eco-conscious brands.
Meghan topped the table for sustainable fashion, but has no links to environmental charities, according to the firm.
Prince Charles on the other hand has ties to 69 environmental not-for-profits.
The Queen herself was mid table, scoring an overall total of 23, six points behind Kate but 14 ahead of the Sussexes.
Overall the CO2 footprint caused by the royal family traveling, between 2018-2019, was 130 tonnes in total, 77% higher than this year.
It comes as Prince William joined forces with Sir David Attenborough to launch a new prize for environmentalism.
With £50m to be awarded over a decade, the "Earthshot Prize" is the biggest environmental prize ever.
The Prince said "positivity" had been missing from the climate debate - something the award could supply.
Representatives of the Duke and Duchess of Sussex have been contacted for comment.