Nasceu em Massarelos, na cidade de Porto em 1844, filho de John Cassels, Diogo era o primogénito de uma numerosa família, pois contava seis irmãos e seis irmãs; foi criado no porto e em Vila Nova de Gaia, seguindo mais tarde para Inglaterra onde estudou no colégio de Repton. Por falta de saúde, aos 14 anos, teve de abandonar o colégio e regressar a Gaia, continuando depois durante algum tempo o estudo das línguas inglesa, francesa, latina,e grega, com professores particulares.
Obrigado a deixar os estudos ainda muito novo, para começar a trabalhar, a fim de auxiliar a família, tornou-se mais tarde sócio da firma John Cassels. Fundada por seu pai. Em virtude da morte desde foi durante alguns anos gerente de fabrica de estamparia da mesma sociedade, em Mafamude, que também tinha sido montada por seu pai. Este primeiro período da sua vida foi de intensa actividade, pois teve de trabalhar com afinco, visto sua mãe e seus dez irmãos terem ficado a seu cargo. Tempos depois, resolveu passar essa sociedade e dedicar-se ao comércio, fundando a casa comercial James Cassels, no Porto, a qual ainda hoje existe, apesar de ser dirigida por pessoas estranhas a sua família.
Estes acontecimentos da sua vida foram descritos por ele próprio, numa carta aos vilanovenses. Publicada no número 275 do periódico gaiense a Igreja Lusitana.
Quando tinha 24 anos de idade, Diogo Cassels fundou a escola do Torne, para classes pobres.
Em 1885 fez exame do magistério na escola normal, no Porto, e em 1891 obteve o diploma de professor de instrução secundária. Mais tarde ou seja em 1901, com o produto dum seguro de vida, fundou a escola do prado, leccionando e dirigindo ambas as escolas com inexcedível dedicação e competência.
Dedicou incansavelmente a sua actividade à difusão da instrução e da educação das classes humildes, estimulando-as e incutindo-lhes a veneração por todos os sentimentos nobres que dignificam o homem, ensinando e instruindo, sem desfalecimento, gastando, nessa missão, a sua grande fortuna.
Em abril de 1908, a liga nacional de instrução reconhecendo os grandes serviços prestado por diogo cassels no combate ao analfabetismo, concedeu-lhe o diploma de "benemérito da instrução".
Homenagem da câmara municipal de Gaia
A câmara municipal de gaia, na sua sessão realizada no dia 20 de outubro de 1910, resolveu, por unanimidade (o que já tinha sido resolvido pela sessão transacta) dar o nome de Diogo Cassels à antiga rua do torne, artéria central que, como se sabe atravessa a avenida da república, ligando a rua 14 de outubro com a rua general torres.

Falecimento
No dia 7 de novembro de 1923, quando Diogo recebia um donativo para as escolas, no banco inglês, foi acometido, repentinamente, por um ataque que o prostrou para sempre.
No funeral do insigne educador, a câmara municipal reuniu extraordinariamente ao tomar conhecimento da morte do maior homem do concelho, instituindo por unanimidade, dois prêmios de Esc. 100$00 cada, denominador Diogo Cassels e Isabel Cassels", para serem distribuídos anualmente, ao aluno e aluna mais distintos das escolas fundadas pelo grande benemérito. Em 10 de abril de 1938, foi inaugurado solenemente o monumento a Diogo Cassels, no jardim-parque da Avenida da República em Vila Nova de Gaia.

Diogo Cassels was born at Massarelos, Porto in 1844. He was a son of John Cassels and the first of a large family of six brothers and six sisters. He grew up in Porto and Vila Nova de Gaia. Later he went to England to study at the Repton College. Because of health problems, at the age of 14 he saw himself forced to renounce his studies and college, and he came back to V. N. de Gaia, but keeping studies in English, French, Latin and Greek with private teachers.
Even so he was, once again, forced to give up his studies to start working, to help his household. Later he became partner at his father’s company. When his father died he took the responsibility of being, for some years, the manager in a stamped tissues factory, also founded by John Cassels, and part of the same enterprises group at Mafamude. This first period of his life was of much activity and hard work because the responsibility for his mother and 10 brothers and sisters relied in his shoulders.
Sometime later he decided to sell this Corporation he inherited from his father, to dedicate himself to trade. He opened another Firm called “James Cassels” at Porto that still exists but now without any connections with the Cassels family.
All his life’s happenings are largely described by him in a letter he addressed to the People of Vila Nova de Gaia, which was published in the Newspaper of the Lusitanian Church with number 275 of this newspaper Collection.
When he was 24 years old, James Cassels founded the Torne Primary School, for the poor. In 1885 he himself was submitted to an test at the official Public School in Porto, to obtain the Diploma that gave him the possibility to be recognized as a secondary education teacher. Later, in 1901, with the money of his Life Insurance, he decided to open another Primary School, this time at the Prado Church. He was the Headmaster and teacher of both Schools and everyone recognized in him the gifts of complete dedication and competence.
He dedicated his life without resting to activities with the objective of spreading education among the people of poor social standards, stimulating and educating everyone in the noble values that dignifies humankind, teaching, without giving up, and spending all his wealth in this mission.
In April 1908, the Instruction National League for Education, recognizing the great services he accomplished against illiteracy, and attributed him the “National Education Meritorious Award”. This was a Vila Nova de Gaia City Council decision.
This same City Council, at its General Assembly Session in 20th October 1910, with an unanimous vote decided to give his name to a street, becoming the “Rua Diogo Cassels”. Previously this street was called “Rua do Torne”. Still is one of the main roads that crosses with the most important Gaia Avenue, connecting the “Rua 14 de Outubro” with “Rua General Torres”.
Death.
In November, 7th 1923, when Diogo Cassels was accepting a donation for the Schools at the British Bank, he suffered a serious stroke and later on died. Immediately before his funeral the City Council was called officially to mourn Diogo Cassel´s death, and they decided in that same day to create, unanimously approved, two Scholarships Prices of 100.00$ each. The Prices were called “Diogo Cassels” and “Isabel Cassels” Scholarships. They were attributed on a year basis to the best and distinguish student from the Schools he founded – one girl and one boy. On 10th April of 1938, was solemnly dedicated to him a monument, at Avenida da República Garden-park, Vila Nova de Gaia.