Concise Bible Dictionary:
See GRACE.
From Anstey’s Doctrinal Definitions:
Mercy is “not getting what we deserve”—i.e. being judged for our sins. It is often mentioned in contrast to grace, which has to do with “getting something we don’t deserve”—i.e. salvation and the many blessings associated with it. Thus, God extends His “mercy” to men in their sins (Eph. 2:4) because He does not want any to perish in a lost eternity (2 Peter 3:9).
On the other hand, the "mercies" of God have to do with temporal things that God grants to men providentially in the vicissitudes of life on earth (Gen. 32:10; Psa. 40:11; Lam. 3:22; 2 Cor. 1:3; Phil. 2:1; Col. 3:12). This would be things having to do with His care for people amidst the dangers of life, etc. Thus, in contrast to our blessings which are heavenly, spiritual, and eternal, the mercies of God are earthly and temporal provisions that He grants to men on earth. This distinction between mercy and mercies is a general thing in Scripture; there may be exceptions. (“Mercies” in Romans 12:1 should be translated “compassions”).
The following quote from The Christian Friend magazine (E. Dennett, editor) is helpful in this regard: "In verse 3 [of Ephesians 1] we find the best kind of blessings, in the best place; they are in Christ, and they are all there. We have temporal mercies assuredly, but that is just a kind of passing thing, and not our portion" (The Christian Friend, vol. 9, 1882, pp. 213-214).