Marconi, the second son of a prosperous Italian country gentleman and a wealthy Irishwoman, was born in Bologna, Italy, and educated there and in Leghorn (Livorno). He studied physics under several well-known teachers and had the opportunity of learning about the work carried out on electromagnetic radiation by Heinrich Hertz, Oliver Lodge, Augusto Righi, and others.

Marconi became interested in using Hertz's ‘invisible rays’ to signal Morse code and in 1894 began experimenting to this end at his father's estate. Similar work was being done at the time in Russia by Aleksandr Popov. Although he convinced himself of the importance of this new system and was soon able to transmit radio signals over a distance of more than a mile, he received little encouragement to continue his work in Italy and was advised to go to England.

Shortly after arriving in London in February 1896 Marconi secured the interest of government officials from the war office, the admiralty, and the British postal service. The next five years he spent demonstrating and improving the range and performance of his equipment, and overcoming the prevailing skepticism about the usefulness of this form of transmission. In 1897 he helped to form the Wireless Telegraphic and Signal Co. Ltd., which in 1900 became Marconi's Wireless Telegraph Co. Ltd. He achieved the first international wireless (i.e., radio) transmission, between England and France, in March 1899; this aroused considerable public interest and attracted attention in the world's press. In the same year the British fleet's summer naval maneuvers, for which Marconi equipped several ships with his apparatus, helped to convince the admiralty and mercantile ship owners of the value of radio telegraphy at sea.

In December 1901 Marconi succeeded in transmitting radio signals in Morse code for the first time across the Atlantic, a distance of some 2000 miles (3200 km). Already well known, Marconi created a sensation, became world famous overnight, and silenced many of his critics from the scientific world who had believed that because radio waves travel in straight lines they could not follow the curvature of the Earth. This phenomenon was explained by Arthur Kennelly the following year as being due to the presence of a reflecting layer – the ionosphere – in the Earth's atmosphere. Thus by 1901 radio telegraphy had become a practical system of communication, especially for maritime purposes. Marconi spent the rest of his life improving and extending this form of communication, and managing his companies.

Although a good deal of Marconi's work was based on the ideas and discoveries of others he was granted various patents and was responsible for some notable inventions. These included the first of all patents on radio telegraphy based on the use of waves (1896), the elevated antenna (1894), patent 7777, which enabled several stations to operate on different wavelengths without mutual interference (1900), the magnetic detector (1902), the horizontal directional antenna (1915), and the timed-spark system of generating pseudo-continuous waves (1912).

From about 1916, Marconi began to exploit the use of radio waves of short wavelength, which allowed a more efficient transmission of radiant energy. In 1924 the Marconi company obtained a contract to establish short-wave communication between England and the British Commonwealth countries and by 1927 a worldwide network had been formed. In 1932 Marconi installed a short-wave radio telephone system between the Vatican and Castel Gandolfo, the Pope's summer residence.

Despite having little interest in anything outside radio Marconi several times acted in an official capacity for his government: he was sent as a plenipotentiary delegate at the 1919 Paris peace conference. In 1923 he joined the Fascist Party and became a friend of Mussolini. Marconi received several honorary degrees and many awards, which included the Nobel Prize for physics jointly with Karl Braun (1909), being made a marquis (marchese) in 1929, and president of the Royal Italian Academy (1930). At his death he was accorded a state funeral by the Italian government. All Post Office wireless telegraph and wireless telephone services in the British Isles observed a two-minute silence at the hour of his funeral.