Mnuchin "never seems to consider flying commercial," said Jordan Libowitz, spokesman for Citizens for Responsibility and Ethics in Washington, the group that issued the report.
"He's always looking for the biggest and best private and military plane," Libowitz said of Mnuchin, a former Goldman Sachs partner.
CREW's report, which is based on details of Mnuchin's travel obtained through Freedom of Information Act requests, says that "between the spring and fall of 2017, Mnuchin took seven separate trips on military aircraft at a total of nearly $1 million."
"Those records make clear that Secretary Mnuchin has legitimately earned a place in the rogues' gallery of cabinet secretaries who have abused their all too easy access to military and other non-commercial aircraft for both business travel and what, upon closer inspection, appears to sometimes include personal travel," CREW said in its report.
The report cited records for Mnuchin's predecessors during the decade before he became Treasury secretary, which show that for 10 wholly domestic trips, seven trips totally or partially used commercial planes, while just three used more expensive military planes.
"The documents Treasury provided CREW show that to date Secretary Mnuchin apparently has not made a single trip on a commercial aircraft," the report said.
A Treasury Department spokesman said in an emailed statement to CNBC: "Much of CREW's 'report' consists of falsehoods and mischaracterizations."
"Even CREW concedes that the Secretary's travel requests 'bear a remarkable similarity' to the requests submitted by secretaries in the Obama Administration, using the same approval process and level of justification."
"The documents CREW cites are the same documents that were examined by Treasury's Inspector General in his review of Treasury's travel, in which he identified no violation of law, regulation, or ethics requirements. The Inspector General confirmed that the Secretary is 'required by applicable authority and policy to have access to secure communications at a high level of classification at all times,' justifying the use of military aircraft. These documents explicitly demonstrate Treasury's concern for being prudent with taxpayer dollars while fulfilling important departmental responsibilities."
In a later statement, Tony Sayegh, assistant secretary for public affairs at the Treasury, said: "Secretary Mnuchin's 2017 official travel totaled $1.2 million, substantially less than the $2.8 million his predecessor in the Obama administration averaged annually. Both figures represent the cost to the government for commercial and government air travel, train travel, hotels, vehicles, and other costs for the Secretaries and their security and traveling staff."
CREW started an inquiry into Mnuchin's travel last August after a controversy exploded on the heels of a trip he took to Fort Knox, Kentucky, on a military plane with his wife, actress Louise Linton. That trip, it was revealed in the report, cost $33,046.
Linton had lashed out on Instagram against a woman who had criticized her post on that social media platform that highlighted the designer clothes she was wearing during that trip.
CREW filed a FOIA request for details of that trip, and said it seemed to have been timed to view the solar eclipse in Kentucky, a prime location to see that rare event.
"As part of an administration supposedly committed to draining the swamp, why did the treasury secretary need a military jet to transport himself and his wife, when a readily available commercial flight would have saved thousands of dollars in taxpayer money and likely gotten them to their destination just as quickly?" CREW said in its report.
Among the trips cited was one in May 2017 to Bari, Italy, for a two-day meeting of Group of Seven finance ministers. Treasury originally requested a C-40 aircraft, which the report noted can accommodate as many as 120 passengers.
The trip, which ended up costing almost $314,000, wound up being taken on a smaller plane that can carry up to 45 passengers, the report said. But only 24 people, including Mnuchin, were listed on the flight manifest, CREW said.
The group's report noted another trip which involved just a 1½-hour flight to Ottawa, Ontario, for a meeting and news conference with Canada's finance minister, on June 9, 2017. Twenty people, including Linton and several members of the media, flew on a military aircraft at a cost of $16,350, the report said.
CREW pointed out that the justification for that plane and its secure communications system during that short flight was justified in the request for the aircraft by "the potential for developments during travel related to a number of issues."
In October, the Inspector General's Office of the Treasury Department issued a report that found that Mnuchin had not violated the law in his use of military aircraft for his travel on seven occasions.
However, the same report said it had a "concern" about "a disconnect" between the standard for proving that a trip on military aircraft by senior executive branch officials is justified as being for a so-called White House Support Mission, and the "actual amount of proof provided by Treasury and accepted by the White House in justifying these trip requests."
A trip Mnuchin took to Miami on June 15, 2017, with four staffers and two Secret Service members used a military version of the Gulfstream 550 jet at a cost of $43,725.50, according to the IG's report. That plane holds up to 19 passengers.
The same IG's report noted that the cost per person on a commercial flight would have been $688 for a round trip, or $4,816 in total.
Anne Weisman, CREW's chief FOIA counsel, said: "From the documents we obtained" after the IG's report was issued, "it appears Secretary Mnuchin considers first and foremost his own comfort and ease, leaving the protection of taxpayer money at the bottom of his list of priorities."
"The public still has no reasonable explanation for why Secretary Mnuchin apparently has never used commercial aircraft while his predecessors did, or why he needs military aircraft that can accommodate 120 passengers when his travel manifests contain far fewer names," Weismann said.