Born to Francis Ronalds and Jane (née Field), wholesale cheesemongers, at their business premises at 109 Upper Thames Street, London, he attended Unitarian minister Eliezer Cogan's school before being apprenticed to his father at the age of 14 through the Drapers' Company.

The family later resided in Canonbury Place and Highbury Terrace, both in Islington, at Kelmscott House in Hammersmith, Queen Square in Bloomsbury, at Croydon, and on Chiswick Lane.

[5] His brother Hugh was one of the founders of the city of Albion in the American Midwest,[6] and sister Emily Ronalds epitomised the family's interest in social reform.

He established that electrical signals could be transmitted over large distances with 8 miles (13 km) of iron wire strung on insulators on his mother's lawn in Hammersmith.

[19] Commercialisation of the telegraph only began two decades later in the UK, led by William Fothergill Cooke and Charles Wheatstone, who both had links to Ronalds' earlier work.

The second one enabled a scene or person to be traced from life onto paper with considerable precision; he and Dr Alexander Blair used it to document the important Neolithic monuments at Carnac, France, with "almost photographic accuracy".

[4][page needed] Ronalds set up the Kew Observatory for the British Association for the Advancement of Science in 1842 and he remained Honorary Director of the facility until late 1853.

[29] He applied his technique in electrographs to observe atmospheric electricity, barographs and thermo-hygrographs to monitor the weather, and magnetographs to record the three components of geomagnetic force.

[27] To observe atmospheric electricity, Ronalds created a sophisticated collecting apparatus with a suite of electrometers; the equipment was later manufactured and sold by London instrument-makers.

Ronalds endeavoured to employ his atmospheric electricity equipment and magnetographs in a detailed study to understand the cause of the anomalies but had insufficient resources to complete his work.

[4] Ronalds' final foreign sojourn in 1853–1862 was to northern Italy, Switzerland and France, where he assisted other observatories in building and installing his meteorological instruments and continued collecting books for his library.

[34] During his last years, however, his key accomplishments became well known and revered in the scientific community, aided in particular by his friends Josiah Latimer Clark and Edward Sabine and his brother-in-law Samuel Carter.

Colleagues at the Society of Telegraph Engineers regarded him as "the father of electric telegraphy",[35] while his continuously recording camera was noted to be "of extreme importance to meteorologists and physicists, and… employed in all first-rate observatories".