As per the traditions of the Holy Prophet of Islam (saw), the Mahdi will be the last in the line of Imams. This is found in 136 traditions of the Holy Prophet (saw).

For the benefit of my readers, a few references for this tradition are given below:
1.   Sharhe' Nahjul Balagha, volume 1, page 206. Sermon 157
2.   Bayane' Shafa'ei, page 506
3.   Iqdud Durar, chapter 1, page 25
4.   Iqdud Durar, chapter 7, page 142
5.   Iqdud Durar, chapter 7, page 145
6.   Majmue' Zawaed, volume 7, page 316-317
7.   Muqaddamae' Ibne Khaudun, chapter 53, page 252
8.   Al Fusulul Mohimmah, part 12, page 297-298
9.   Arful Wardi by Suyuti, volume 2, page 61
10. Sawaequl Mohreqah by Ibne' Hajar Makki, pg 163, chp 11, part 1
11. Tameezut Tayyeb, page 196, tradition number 1493
12. Kanzul Aamaal, volume 14, page 597, tradition 39672
13. Al Burhan by Muttaqi Hindi, page 91, chapter 2, tradition 7, 8
14. Fara'edul Fawaedul Fikr, chpater 1, page 3
15. As'aaful Raghebayn, page 145
16. Nurul Absar by Shiblanji, page 177
17. Al Imamah Wat tabserah, page 92, tradition 71
18. Kamaluddin, volume 1, chapter 22, page 31
19. Amali of Mufid, page 277, speech number 36, tradition 8
20. Amali of Tusi, volume 1, page 63
21. Al Malahim by Sayyed ibn Tawoos, chapter 191, page 76-95
22. Kashful Ghummah, volume 3, page 263
23. Isbatul Huda by Shaykh Hurre' Aameli, volume 3, page 596
24. Hilyatul Abrar, volume 1, chapter 43, page 450
25. Ghayatul Meraam, chapter 161, page 700, tradition 105
26. Beharul Anwar, volume 32, chapter 8, page 298, tradition 257
27. Muntakhabul Asar, chapter 1, part 2, page 152, tradition 32

The Holy Prophet (saw) has clearly indicated that Islam will be completed only by the advent of the Mahdi and through him, Islam will spread across the world and overcome all other religions.

Neither prophecy was fulfilled by the coming of Mirza Ali Mohammed Shirazi, which negates his claim of being the awaited Mahdi of Islam.