The Lausanne-based body, the final court of appeal in sports disputes, ruled that world 400 metres champion Jerome Young should have been banned from the 2000 Sydney Olympics after a positive dope test in 1999.
The Jamaican-born American was a member of the relay squad that won the 4x400 metres final at the Sydney Games. Johnson ran the anchor leg for the United States after retaining his individual 400 title.
CAS said Young should have been banned from June 26 1999 to June 25 2001 after testing positive for nandrolone.
"The consequence of this finding is that Mr Young should not have been eligible to compete in any competition during that period, including the Olympic Summer Games in Sydney in 2000," CAS said.
Young said: "I believe that today's CAS decision is fundamentally unfair -- I was exonerated in 2000 by a panel of three independent and objective arbitrators who considered the evidence before it and concluded that USA Track & Field failed to prove its case against me."
Tuesday's finding will now be considered by the ruling council of the International Association of Athletics Federations (IAAF) at its next meeting in Athens in August.
Last month, the IAAF annulled the British team's second place in the 4x100 metres relay at last year's Paris world championships after Dwain Chambers tested positive for the new designer steroid THG (tetrahydrogestrinone).
A spokeswoman for the International Olympic Committee (IOC) said the IOC would wait for the IAAF's recommendation before deciding what to do about the Sydney medals.
ERRONEOUS JUDGMENT
The American squad in Sydney comprised Young, Johnson, Alvin Harrison, Harrison's twin brother Calvin, Antonio Pettigrew and Angelo Taylor. Young and Taylor did not run in the final.
CAS said it could not consider the possibility of other members of the squad besides Young losing their gold medals.
"It is, however, sufficient to say that the panel does not necessarily accept that, in the unusual circumstances of the present case, this consequence must follow," it said.
"Whether it does or not, is, however, a matter for the IOC and/or the IAAF to consider and not this panel."
Asked about the possibility of losing the gold medal, Johnson said: "I don't know how I would feel, or what I would do. I never felt it was going to happen because I felt there was no legitimate case for them to do that."
The CAS statement said a sample taken at the U.S. championships in Eugene, Oregon, on June 26 had tested positive for nandrolone.
A USA Track & Field (USATF) Doping Hearing Panel had found that a doping violation had been committed but an appeal by Young was upheld by the Doping Appeals Board (DAB).
The DAB then said a negative test produced in Lausanne six days later raised reasonable doubts that a violation had been committed.
"The panel finds that the basis on which the DAB made its finding and the decision to exonerate Mr Young were erroneous," CAS said.
'EACH STEP'
A statement from USATF said the American governing body noted the decision and added it believed it had complied with the IAAF rules "at each step of the way".
IAAF president Lamine Diack said: "The IAAF is extremely pleased that a case that began in 1999 can now finally be closed."
CAS will now consider the case of world 100 metres record holder Tim Montgomery, who has elected to go to arbitration abroad after receiving a letter from the U.S. Anti-Doping Agency (USADA) alleging doping violations.
"We are taking this step because we believe that USADA's conduct...has been so egregious as to remove any confidence that Tim could be fairly treated in this process," Montgomery's lawyer Howard Jacobs told the SanJose Mercury-News.
Montgomery, the partner of triple Olympic champion Marion Jones, has denied committing any drugs offences.
