is an extension of the grammar point てから meaning 'after (A)', and partners と言（い）う 'that which is' with もの 'thing'. It is often translated as 'ever since (A)', and will always express a new state of (B) that has been ongoing since (A). Although the literal translation is 'with that which was from (A)', the fact that (B) is ongoing is simply implied.
As with てから, this structure will be seen following verbs in their て-form.
Ever since I joined this company, I have not been home before the clock ticks over midnight.
Ever since I started to go on walks before work, I have been steadily losing weight, so I am thinking of continuing to go on walks.
Ever since watching the movie 'Final Destination', I have been avoiding driving behind trucks carrying lumber.
Although not quite as common, からというもの
may be seen following それ, when それ is at the beginning of a new sentence. In these cases, それ will linked to what was stated in the previous sentence.
He was yelled at in front of everyone by the boss. Ever since that event, he became a different person.
She was groped last month. Ever since then, she has been avoiding packed trains.
As this phrase implicitly highlights something that has remained ongoing ever since (A), it will not be used when (A) is something that only happened in the very recent past. However, because length of time is subjective, it usually depends on what is considered 'long' based on any specific outcome.