Giorgio de Stefani was born in Verona in 1904, son of a Minister of Government. He learned tennis from his mother on the courts of their family house at Lake Garda. He started his amateur career by competing in and winning the Montreux tournament in 1920 at the age of 15. During his student years he won the world university championships in Darmstadt where he defeated Fritz Kuhlmann without losing a set. Although he and Emanuelle Sertorio lost the doubles final to the German student team of Kuhlmann-H. Uthmoller in five sets. Next year he drafted into the inter-college tennis challenge of Villa d'Este in 1931 featuring the joint Harvard-Yale team against the Italian student players where he won one singles match against Robert Ryan and lost one singles against Bill Breese and two doubles. The Americans defeated the Italians nine rubbers to five. At the age of 22 he debuted in the Davis Cup as a reserve player.
He was the top Italian player five times from 1933 to 1936 and in 1938. He was No. 8 in Europe in 1931 according to the Z√ľricher Sport newspaper and reached the same rank in 1935 on the list of Heinrich Kleinschroth, a former German champion and Davis Cup player.
British Fred Perry and De Stefani met several times in their careers. Their first meeting was in the 1931 French Championships fourth round, where Stefani overcame Perry in four sets. In their 1932 Pacific Southwest Tournament semifinals rematch Perry equalized their tally and went on to face Jiro Satoh for the title. In their third encounter in the 1933 International Lawn Tennis Challenge where Great Britain defeated Italy with Stefani winning the only rubber of that quarterfinal. Their most controversial match came in the 1934 French Championships quarterfinal where Stefani led two sets to one with Perry leading in the fourth to force a deciding set, when the Briton fell awkwardly while running to the net and hurt his ankle. Subsequently Perry tried fix the match with Stefani. He wanted to lose as a noble thus offering his Italian opponent "an honourable victory" if they avoid long, wide rallies that would damage his ankle more. According to Perry it was Stefani who rushed to help him up when this happened. Subsequently Stefani turned and took the fourth and final set, but Perry recalled it as a gentleman affair, where Stefani stuck to their plan all the time. Right after the match Perry collapsed and had to be carried to the dressing room where the doctors diagnosed him with a sprained ankle. Although Perry swore to take revenge on him the next time they play again. Thus in return, on their next match-up in the quarterfinal of the Australian Championships in Melbourne Perry crushed the Italian, ruining his best oversees result and giving him the biggest loss of his life with a score of 6-0, 6-0, 6-0. He also stopped him in the doubles contest quarterfinals as well with a straight sets victory. Perry stated later that "That determination was part of my character". Three years later The Glasgow Herald described their relationship as Stefani being the hoodoo to Perry's career. Perry eventually went on to become the 1934 world amateur number one. Perry torned pro after the 1936 season, while de Stefani got more and more involved in sports diplomacy.
After the outbreak of the war tennis tournaments and sport in general were neglected. As an attempt to reorganize the Italian sports life count Alberto Bonacossa offered him the position of Regent of the Presidential Committee of tennis, which Stefani accepted and held in 1943, however as Italy's war involvement escalated, it forced him to leave his office. He fled from Rome and joined the Italian resistance movement of Breuil-Cervinia. He surrendered to General Harold Alexander with the aid of the partisans, who helped him to make contact with the English forces. He was transferred to the Montreux internment camp thereafter. Fortunately the Mayor of Montreux heard news about his presence and pleaded for him to be released to a hospital.
De Stefani later became a sports official, was invited and elected in August 1951 on the International Olympic Committee (IOC). In 1966 he initiated the reintroduction of tennis at the Summer Olympics at the IOC meeting in Melbourne and eventually succeeded when eleven votes had been discarded. The President of the IOC, oppositional Avery Brundage protested to the outcome and demanded a new voting process due to fabricated technical errors made in the previous one. In the end the initiative was rejected and tennis was only readmitted to the Olympics in 1988. In 1967 when he was the president of the International Lawn Tennis Federation (ILTF) in the debate on the status of Open tennis tournaments, especially accepting Wimbledon as an official Open Grand Slam he sided with the ban of the said tournament threatening the Lawn Tennis Association of getting expelled from the Federation and having its championships cancelled. The next year he opposed to allow professionals to enter into the Davis Cup competitions. Allegedly he even convinced Italian number one Nicola Pietrangeli with financial means to devote himself exclusively to amateur tennis and never turn professional. He was also an abolitionist for the challenge round in Davis Cup and the choice of field by the finalists but failed to succeed in these issues.
He served as chairman of the ILTF in 1955 and 1956, 1962 and 1963, and from 1967 to 1969, and also as president of the Italian Tennis Federation, where he stayed from 1958 to 1969.
Giorgio de Stefani was an ambidextrous player, which allowed him to switch his racquet from one hand to another to always return a forehand shot. As a result, he didn't have a backhand stroke. He was thus unpredictable and even players failed to recognize this trick sometimes even for multiple sets. He could also benefit from this ability in serving when he was able to easily change the serve direction. He asked for a permission to use two racquets simultaneously, one in each hand, but it was officially banned in 1931 by the ILTF. Although when playing golf his preferred shot was with his right hand as well as while serving. In one of his articles written to The Sydney Morning Herald, Fred Perry described Stefani as a "freak player" and his style as unorthodox and that he lacks the beauty of shot execution of those as Bunny Austin and Jack Crawford. He further described his service and his volleying as weak, his physique as medium and his condition well enough to be able to run around the whole match. Thanks to his ambidextrous nature he possessed good passing shot skills. His forehands were solid but they lacked pace although his right hand was somewhat more accurate. Stefani was a hard court specialist accommodated to hot weather.
Apart from playing amateur tennis he was a devoted mountain-climber and occasional golfer. He served his voluntary military service in 1930. His uncle was the politician Alberto De Stefani, who was the Minister of Finance, but was removed by Benito Mussolini due to their ideological differences. Giorgio was awarded the title Knight of the Order of the Crown of Italy for his sports achievements. He graduated at the University of Rome earning a doctorate in law. In 1956 he married Maria Carolina di Marchesi Spinola. His first child died. In his retirement years he received the "Atleti Azzuri" Trophy for his sports merits.