Grandis was born in the county of Nice, at that time part of the Kingdom of Sardinia, from parents originally from Borgo San Dalmazzo (CN).
When Sebastiano was still a child, his father Valentino left the mine and opened a drapery factory in Borgo San Dalmazzo, where he moved with his family.
After studying at the Royal College of Ventimiglia and finishing high school in Cuneo, in 1837 Sebastiano moved to Turin to attend university.
In 1846 he was sent together with Germain Sommeiller, already his fellow student, to Belgium in order to learn the techniques of construction and management of railways.
[5] Upon his death, he left his possessions to the town of Borgo San Dalmazzo, to finance scholarships for deserving young people.