I was previously married and have one daughter from that marriage. After getting a divorce, fifteen years ago, I married again and have a boy and girl from that marriage.
Please tell me whether my current husband is considered a mahram [individual to whom marriage is impermissible] to my daughter from my first marriage?

From the well known principles of Islamic law is the axiom, "affinity prohibits a husband to marry his step daughters with whose mother he consummated marriage." This axiom's application is unrestricted- whether they (the husband and the wife) are married, divorced or the wife is deceased. Allah Almighty said:

And (forbidden to you for marriage are) your step-daughters who reside with you, [born] of your wives of whom you have consummated the marriage.
[Qur`an 4:23]

The prohibition of a girl marrying her stepfather is not contingent on whether she resides with him. Scholars applied the restrictive clause, "your step-daughters who reside with you," from the verse above in the figurative sense. Meaning, that in most cases, a stepdaughter is brought up by her mother under the guardianship of her stepfather, but if this were not the case (if she were raised in her real father's home), the prohibition of marriage still stands.

The ruling

Your current husband is considered a mahram to your daughter from your first marriage.

Allah Almighty knows best.