Mitra is largely paired with varuna in rig veda but is also praised separately. Vishnu takes his three steps by the laws of Mitra. He is also praised as "he supports all the gods".
Mitra-Varuna is the most common dual deity addressed in the Rig Veda. The dual deities Mitra-Varuna are the lords of justice, are said to be laying down the law for men. They watch over oaths and punish oath-breakers They are both considered Ādityas, or deities connected with the Sun; and they are protectors of the righteous order of rta. Their connection is so close that they are frequently linked in the dvandva compound Mitra–Varuna.
Devi Bhagavata Purana -Chapter 14 - On the birth of Vaśiṣṭha from Mitrā Varuṇa - tells us the story of birth of rishi Agasti and Vaśiṣṭha from semen of Mitra-Varuna in an uncovered jar wihout womb. Here we find the mentioning of the pair Mitra-varuna.
53-69 Urvaśī became attached to them; and, under their control, began to stay in the house of Mitrā Varuṇa. When Urvaśī began to remain there, strongly attached to them, the semen of Mitrā Varuṇa dropped in an uncovered jar. And the two beautiful sons of the Ṛṣis were born out of that; Agasti was the first child and Vaśiṣṭha the second.
The same account is also found in Shreemad Bhagvat Purana where Mitra-varuna is mentioned.
वाल्मीकिश्च महायोगी वल्मीकादभवत्किल ।
अगस्त्यश्च वसिष्ठश्च मित्रावरुणयोरृषी ॥SB 6.18.5॥
vālmīkiś ca mahā-yogī valmīkād abhavat kila
agastyaś ca vasiṣṭhaś ca mitrā-varuṇayor ṛṣī
By the semen of Varuna, the great mystic Valmiki took birth from an anthill. Bhrgu and Valmiki were specific sons of Varuna, whereas Agastya and Vasisṭha Ṛsis were the common sons of Varuna and Mitra, the tenth son of Aditi.SB 6.18.5
Here Bhagvat Purana Mitra is mentioned as one of the Adityas among 12 Adityas . Mitra - May - June- Shukra- Jyeshtha
मित्रोsपि: पौरुषेयोsथ तक्षको मेनका हहा: | रथव्स्वन् इति ह्येते शुक्रमासं नयन्त्यमी || SB 12.11.35||
mitro 'triḥ pauruṣeyo 'tha takṣako menakā hahāḥ
rathasvana iti hy ete śukra-māsaḿ nayanty amī
Mitra as the sun-god, Atri as the sage, Pauruseya as the Raksasa, Taksaka as the Naga, Menaka as the Apsara, Haha as the Gandharva and Rathasvana as the Yaksa rule the month of Sukra.SB 12.11.35
Same as above in Vishnu Purana .
Jyeṣṭha they are Mitra, Atri, Hāhā, Menā, Rathaswana, Takṣaka, and Paurusheya.
Mitra as Aditya in The Mahabharata - Adi Parva.
From Aditi have sprung the twelve Adityas who are the lords of the universe. And, O Bharata, as they are according to their names, I shall recount them to thee. They are Dhatri, Mitra, Aryaman, Sakra, Varuna, Ansa, Vaga, Vivaswat, Usha, Savitri, Tvashtri, and Vishnu
Here in the The Mahabharata - Vana Parva
So yes Vedic god Mitra is mentioned in post-rig-vedic-works epics-and-puranas.